The Belgian playmaker limped off with only five minutes remaining on Thursday night, raising fears that he would have suffered a recurrence of the problem which saw him miss the start of the season.
After the match, City boss Pep Guardiola said he had no idea how long De Bruyne could be missing but reports this morning suggest he will be out for around five weeks, meaning he will be out of the Manchester derby.
De Bruyne was injured after Fulham defender Timothy Fosu-Mensah landed awkwardly on his left knee.
Reports today suggest the player will be out for up to six weeks so as well as missing out against Manchester United, he'll be unavailable for the Champions League games against Shakthar Donetsk and Lyon. In the Premier League, he will be out for the games against Southampton on Sunday and also West Ham and Bournemouth.
The game against Fulham was the player's first domestic start of the season. During pre-season, De Bruyne suffered an injury to his right knee.
Although City coped without their supremely talented playmaker, the injury comes at a time of the season packed with fixtures in the run-up to Christmas. Guardiola is expected to give a full assessment of the injury at his next press conference but he will clearly be devastated to be missing such a talented player for any amount of time.