De Bruyne suffered lateral knee ligament damage in 2016 after catching studs in the Etihad pitch against Everton and missed nearly 10 weeks of action.
The midfielder has not started either of City's matches so far this season, having returned late from the World Cup after helping Belgium secure third place in Russian Federation.
If the injury is similar to the one he picked up against Everton, he could be out until mid-October at the earliest, although no time frame, in terms of a return date, is known at this time.
He missed two months of the 2015/16 Premier League season after that injury; City will be hoping that this injury is less serious.
Due to being given three weeks off following the World Cup he did not play a role in the Community Shield win against Chelsea but came on for a cameo role of half an hour in the 2-0 opening Premier League victory over Arsenal last Sunday.
De Bruyne was pictured healthy and happy during today's City training session which involved an intense gym workout for the whole squad.
City do, however, have David Silva, Bernardo Silva and Gundogan who can offset his loss in midfield.