Barcelona striker Robert Lewandowski will miss the Clasico against Real Madrid on Thursday with a hamstring strain as the club's problems continue to grow.

Lewandowski, 34, was injured in the shock 1-0 defeat to Almeria on Sunday as Barca missed the chance to open up a 10-point lead over Madrid at the top of LaLiga.

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A source told ESPN he will miss this week's clash at the Bernabeu in the first leg of the Copa del Rey semifinal and potentially league games against Valencia and Athletic Club.

Barca then host Madrid in LaLiga at Camp Nou on March 19, when Lewandowski will hope to be back.

Barca said in a statement on Monday: "Robert Lewandowski has strained the biceps femoris in his left hamstring. The Polish striker is unavailable until the problem clears up."

He missed three games through suspension in January, and Barca have no natural backup for the striker having allowed both Pierre-Emerick Aubameyang and Memphis Depay to leave the club.

Barca are also without Pedri and Ousmane Dembele through injury.

Dembele has not played since hurting his thigh at the end of January against Girona, and Pedri has not featured since picking up a knock against Manchester United on Feb. 16.

Both players hope to be back for the LaLiga Clasico against Madrid on March 19, but Barca are facing a shortage of attackers in the games leading up to that fixture.

In addition to the injuries to Lewandowski, Dembele and Pedri, they will also be without Gavi and Raphinha against Valencia this weekend. Both players received their fifth booking of the season against Almeria and will serve a one-game suspension -- as will coach Xavi Hernandez.

Barca's surprise defeat against Almeria was just their second loss in LaLiga this season and ended a 13-game unbeaten run in the league. They remain seven points clear of Madrid, but the gap could have been 10 after Los Blancos were held to a 1-1 draw with Atletico Madrid on Saturday.

The defeat came at the end of a disappointing week for the Blaugrana. They were also knocked out of the Europa League, losing 4-3 on aggregate to United.

An angry Xavi branded the performance at Almeria the "worst of the season" but, speaking before the game, talked up his side's chances of ending the season with a league and cup double having already won the Spanish Supercopa in January.