The Belgian midfielder was stretchered from the pitch during the Blues' 4-1 pre-season friendly win over a Malaysia XI in Kuala Lumpur but Mourinho has allayed fears that the problem may keep him out for a long spell.
"When he was on the floor and asking for a change, everybody thought the worst but the first assessment by the doctor is that [it is not] ligaments and, if it's something in the meniscus, it has to be a very small injury, so we believe it is not an important injury and hopefully it isn't because the kid is a fantastic player," Mourinho said.
"[He is] a kid but a man on the pitch. Hopefully it's nothing important and he will be ready soon. The doctor wants to do some control [on Monday], and, after that, if necessary, he will go to London.
"But the doctor's feeling - and I trust him a lot because he is very experienced - is that it's nothing important and during the next week here he can recover."