The Netherlands international on Thursday allayed fears that he is set for another long absence after suffering a setback in Sunday's 1-1 draw with Fiorentina.
That was only the 24-year-old's sixth Serie A appearance since returning from an anterior cruciate ligament injury, which he sustained in March last year.
Strootman has now stated that further damage was discovered during his operation.
He tweeted: "The scar tissue from my previous operation has been removed successfully today. Unfortunately some cartilaginous damage, caused by the knock in the Fiorentina match, was found too.
"This means I have to take a longer rest period than anticipated earlier and I am extremely disappointed that it will take much longer before I can be back on the pitch again."
