The 28-year-old was stretchered off during the match at Craven Cottage following a challenge with striker Dimitar Berbatov.
Scans undertaken on Monday have revealed the extent of the injury to his right knee, although Latics manager Roberto Martinez is relieved it is not as bad as first feared.
"Nothing can disguise the fact this is a massive blow because Ivan is a key member of the group but we are relieved that the injury is not as severe as we feared it could have been," Martinez told the Wigan official website.
"The most important thing now is to think of Ivan's welfare, get the operation completed, and for him to begin his rehabilitation.
"Ivan is a very strong and positive character and he is determined to return to fitness as soon as he can.
"He will receive the best possible medical rehabilitation as the club ensures to have him return as an even stronger footballer."
Ramis, who joined Wigan in the summer from Real Mallorca, has been an important player for the Latics this season, making 19 appearances and scoring three goals.