The 21-year-old will have surgery on Tuesday after tearing a knee ligament in the weekend defeat to Hannover
The Bosnia-Herzegovina international was forced from the field 16 minutes into Saturday's 2-1 defeat at the hands of Hannover in Schalke's opening Bundesliga match of the season.
Subsequent tests revealed a tear and Kolasinac is expected to undergo surgery on Tuesday, according to a statement released by the club via their official website.
The 21-year-old, who featured twice for his country at the World Cup in Brazil, made 24 league appearances for Schalke last season.
Kolasinac was quoted during the close-season as saying he had been offered a move to the Premier League with Manchester United.
However, a transfer failed to materialise, with Louis van Gaal ultimately moving to sign Argentina international Marcos Rojo in a €20 million deal from Sporting.
Schalke take on reigning Bundesliga champions Bayern Munich next Saturday in their second game of the new season as they look to bounce back from a disastrous start to 2014-15.
Jens Keller's side were stunned by Dynamo Dresden in the DFB-Pokal before Saturday's defeat by the same scoreline to Hannover, but general manager Horst Heldt insists it is too early to panic.
"If you lose twice in a row, then it is something you can unfortunately not avoid," he said. "We must stand [together] as Schalke.
"But what is important is what is being discussed internally with us. We do not discuss the coach.
"The good thing about football is that a season is not already decided after the first matchday."