The Belgium international failed to impress during his first season at the club after he was bought from Everton for £27m by former manager David Moyes last year.
After spending a stint on the sidelines with injury, Fellaini has made an impact in Louis van Gaal's team in their last two Premier League fixtures.
"The first season, he didn't really find his feet, but I think he knows what it's all about now and the last couple of performances have been very good," the Manchester Evening News quotes Robson as saying.
"He's found where he is. He knows what's expected now and he seems to have settled down and is playing the football he was playing for Everton."
Fellaini could play a part in this Sunday's derby against Manchester City at the Etihad Stadium.