That was confirmed by the Belgian Football Association after he linked up with the national team.
"Fellaini suffered a small tear at the juncture of muscle and tendon in the knee. The injury was revealed during a scan," a statement on the Belgian FA website said.
"Fellaini was injured last Saturday during the match between his club Everton and Wigan Athletic. He will be out three weeks.
"It has been agreed with his club Everton FC he will rehabilitate in Belgium during this period."