Betis suffer Contreras KO
The Spanish international custodian actually sustained the problem in the league encounter against former club Real Madrid a week ago.
The 32-year-old twisted his ankle during the first half of the 1-1 draw at Estadio Ruiz de Lopera and, although he played on for the rest of the match, the knock got worse during the course of last week.
Contreras was forced to sit out Betis' win at Espanyol on Sunday, but was expected to start training again on Monday.
However, further tests have revealed he has damaged ligaments and the player will now be sidelined for at least three weeks.