The Belgian FA said yesterday that the 29-year-old "will be out for a couple of weeks" after an MRI scan revealed that he had sprained his left medial ligament.
Fellaini was forced off the pitch after just 30 minutes in his side's 4-3 victory over Bosnia and Herzegovina in World Cup qualifying last week.
Fellaini has been in inspired form this season - netting four times in eight games - including a brace against Crystal Palace in a 4-0 drubbing at Old Trafford recently.
"Damage could be very little or very big".
As per the Belgium boss, he has received numerous phone calls at 3AM, asking him to keep Lukaku out of his squad for tomorrow night.
"We don't know at this stage (how long Fellanini will be out for)".
That's also good news for Manchester United who will want the former Everton striker ready to take on Liverpool at Anfield on Saturday in the Premier League.