Ilkay Gundogan will undergo surgery after Manchester City confirmed the Germany midfielder damaged the cruciate ligament in his right knee during Wednesday's win over Watford.
The news is a major blow for Pep Guardiola with Gundogan set to miss the rest of the season.
City's £20million summer signing arrived at the Etihad in the summer still recovering from surgery on a dislocated kneecap cap, and injured the same joint in a challenge with Watford's Nordin Amrabat.
Gundogan played on after treatment but it quickly became apparent that the injury was more serious and he limped off before half-time in City's 2-0 win.