Atletico Madrid says midfielder Augusto Fernandez suffered a partial tear of ligaments in his left knee during its 2-1 defeat at Barcelona on Saturday.
It also says that defender Diego Godin twisted ligaments in his left knee during the match.
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Atletico says Fernandez, who arrived this winter to substitute the injured Tiago Mendes, will undergo rehabilitation.
The Argentine was carried off the Camp Nou pitch in the 75th minute.
The club says Godin suffered his injury late in the match. The Uruguay center back is a key player for coach Diego Simeone.
It didn't give expected recovery times for either player.
The loss left Atletico three points behind leader Barcelona in second place.