The Leeds boss has been speaking to the club's media team.
The German has, however, heaped praise on Bianchi, whom he was looking forward to seeing in competitive action.
The Italian's injury is similar to that of Luke Murphy's, although Rosler confirmed that Lewis Cook's injury problems are now past him.
Rosler told LUTV on Friday afternoon: "Unfortunately, we have to announce that Tommy Bianchi will be operated on on Monday. He has a similar problem to what Luke Murphy had: a cartliage operation.
"It happened during a duel against Frankfurt. I felt the player did very well and got stronger and stronger. I was really looking forward to seeing him in competitive action but he will be back in four to six weeks.
"We only have two central midfielder players [fit] so we are not ready for the start of the season. Lewis Cook is fine."
As Rosler states, Bianchi's injury is indeed an unfortunate one - especially considering Leeds' lack of cover in central midfielder currently.
But the German's praise of the Italian suggests he very much rates him ahead of the coming campaign. The 26-year-old has been linked with a move away from Leeds this summer by Vicenza Today but Rosler's comments suggest the Yorkshire side will not be letting him go anywhere.
The news that Cook is okay, meanwhile, is certainly a boost ahead of the coming season, although the Whites' midfield looks as though it won't be fully ready for the new campaign for at least a few weeks.