"Unfortunately, the worst fears have been confirmed: it's a torn cruciate ligament," he wrote on his Facebook page.
"I have the operation on Thursday and straight away THE FIGHT FOR MY COMEBACK BEGINS!"
The 28-year-old suffered the injury just two minutes into Borussia's 2-2 draw at home to Augsburg on Saturday. He is the the second Dortmund star sidelined with a long-term knee injury this season after centre-back Neven Subotic suffered the same fate in November.
"We have only bad news," said Dortmund coach Jurgen Klopp, whose side have picked up just five points in their last seven league games. They are 14 points behind leaders and defending champions Bayern Munich.
"For us this is a disaster, but mainly it's terrible for Kuba (Blaszczykowski). Something like this seems to fit into our season so far, but there has to be a cut-off point (to the injuries) eventually."
Klopp had to patch together a team for most of December as at one point he lost his entire back four to injury.
He is still without injured pair centre-back Mats Hummels and midfielder Ilkay Gundogan.
Last season's finalists Borussia Dortmund breathed life into their hopes of reaching the Champions League knock-out phase with a nervous 3-1 home win over Napoli on Tuesday.
An early Marco Reus penalty and second-half strike by Jakub Blaszczykowski put Dortmund 2-0 up with an hour gone before Lorenzo Insigne pulled one back for Napoli with 20 minutes left. Replacement Pierre-Emerick Aubameyang came on to grab Dortmund's third, 12 minutes from time, to calm nerves and put Borussia level on nine points with Napoli in the group behind leaders Arsenal.
The victory at Dortmund's Westfalenstadion avenged Borussia's 2-1 defeat at Napoli in September and gives them hope of reaching the last 16 following their 1-0 home defeat by Arsenal three weeks ago. With the Gunners three points clear, who will qualify from Group F will now be decided by the final round of pool matches when Dortmund travel to Marseille and Napoli host Arsenal in a fortnight.
Having lost his entire back four to injury, Borussia coach Jurgen Klopp moved defensive midfielder Sven Bender to center-back alongside Greece international Sokratis with his first-choice pairing of Mats Hummels and Neven Subotic ruled out. Dortmund got an early breakthrough when Federico Fernandez hauled down Poland striker Robert Lewandowski in the area and Germany's Reus stepped up to drill home the penalty after 10 minutes.
Just eight minutes later, Reus forced Napoli goalkeeper Pepe Reina into a diving save when he curled in a free-kick. The first half was a nervy affair and with half an hour gone Napoli hit the post when Jose Callejon junked inside Bender and beat Dortmund goalkeeper Roman Weidenfeller.
Borussia Dortmund proved too strong for Schalke in Saturday's Ruhr derby in the Bundesliga as Bayern Munich edged out Hertha Berlin to stay top of the table.
Beaten twice by their bitter rivals last season, Jürgen Klopp's side gained revenge with a 3-1 win at the Veltins Arena, while Bayern came from behind to beat Hertha 3-2 and Bayer Leverkusen also recovered from the loss of the opening goal to win 2-1 against Augsburg.
Bayern remain top of the table, a point clear of Dortmund and Sami Hyypia's Leverkusen. And a clear gap has now opened up below, with Hertha in fourth a further 10 points adrift.
Dortmund got off to a dream start in Gelsenkirchen, taking the lead on 14 minutes when Pierre-Emerick Aubameyang stabbed the ball home at the end of a sweeping move.
Schalke were looking to make it three consecutive wins against Borussia for the first time since the 1960s and they were handed a route back into the game on the half-hour mark when Neven Subotic brought down Christian Fuchs inside the area and referee Knut Kircher pointed to the spot. However, Kevin-Prince Boateng - who once had a spell with Dortmund earlier in his career - saw his penalty saved by Roman Weidenfeller, and Nuri Sahin went on to double the visitors' lead shortly after half-time.
Schalke pulled a goal back when 18-year-old Max Meyer found the net within moments of coming on. It was his third goal in as many Bundesliga games, but Schalke could not find an equaliser and another substitute, Jakub Blaszczykowski, made it 3-1 in the 74th minute at the end of a brilliant counterattack led by Henrikh Mkhitaryan.
Friday evening’s 1-0 defeat to Ukraine in Kharkiv might have put an end to Poland’s World Cup qualification hopes, but as they head to England to play at Wembley Stadium on Tuesday, two of the country’s Borussia Dortmund stars have warned that their own elimination does not mean England are in for an easy game.
Reports on Sunday suggest that around 18,000 Polish supporters will be at Wembley on Tuesday, which will undoubtedly create a raucous atmosphere for this game between two countries who regularly end up in each other’s qualifying group.
Speaking to Przeglad Sportowy, a Polish sports newspaper, national captain Jakub Blaszczykowski and club colleague Robert Lewandowski were bullish ahead of the game. Blaszczykowski alluded to the likely atmosphere inside Wembley on Tuesday, saying, "There are sure to be many Polish fans there. We want to thank them with a good game and the right result."
"It doesn't matter that we only have honor to play for. Matches at Wembley are always very important to us."
Lewandowski also acknowledged the fans, adding that England’s momentum following their comprehensive 4-1 defeat of Montenegro makes them obvious favorites for the game. The Dortmund forward, supposedly Bayern Munich bound next summer despite rumors in recent days that Real Madrid are planning a January bid for his services, said, "We know we have a lot of fans out there who will come to the game. We don't stand a chance of going through, but we want to go to Wembley and at least allow our supporters a little fun. For sure we won't be favorites."
Lewandowski also admitted his frustration at Poland’s failings throughout their qualifying campaign, which has seen them struggle terribly despite having a number of players that are highly regarded individuals across Europe.
He continued, "Sometimes I wonder exactly what we are missing. I am thinking in particular of a few draws in the matches we should be winning. There are moments when it seems that everything is going in the right direction.
"I don't have the right word about what should be changed. Against Ukraine it seemed that we created more chances and we played well, but it is not important. In qualification, victories and points are the most important things, not the style of play."
Poland coach Waldemar Fornalik is unlikely to have his contract extended following this disappointing campaign, and Tuesday is likely to see his players try to send him out with what would be a rare high point in recent months.
Bayern Munich defender Rafinha was given a two-match ban and fined 15,000 euros (US$19,737) by the German Football Federation (DFB) on Wednesday after being sent off in last weekend's draw with Borussia Dortmund.
The 27-year-old Brazilian was dismissed for elbowing Borussia midfielder Jakub Blaszczykowski in the face during the second-half of the Champions League final dress-rehearsal which ended in a 1-1 league draw in Dortmund.
The DFB disciplinary committee gave Rafinha an added fine and an extra game, to the statutory one-match ban for a red card, after the Brazil international also poked Blaszczykowski in the face as he walked off.
The incident led to a pitch side shouting match between Dortmund coach Jurgen Klopp and Bayern's director of sport Matthias Sammer.
Bayern can appeal the ban which sees Rafinha suspended for the remaining two German league games of the season, at home to Augsburg on Saturday and next weekend at Borussia Moenchengladbach.
Bayern have already won this season's German league title and face Dortmund at Wembley on May 25 for the Champions League trophy.
The Bavarians are bidding to become the first German club to win the treble of European, league and cup titles having also reached the German Cup final where they face VfB Stuttgart on June 1.
“On a scale of one to ten, this piece of news is on point nine,” answered Jürgen Klopp, Borussia Dortmund manager, after getting asked about how much the published Mario Götze transfer to FC Bayern Munich commoved his team, hours prior to its big semi-final game against Real Madrid.
Sitting besides Klopp sat Jakub “Kuba” Blaszczykowski, a player who has played for Dortmund for six years and has seen players like Shinji Kagawa and Lucas Barrios depart, while seeing fellow countrymen like Robert Lewandowski and Lukasz Piszczek arrive.