The 29-year-old joined the Red Devils from Arsenal in the summer and has scored seven goals in eight competitive matches for United so far this season.
"The refreshing thing for me is he's doing extra work in training because he's not had any pre-season games," said Ferguson.
"It's quite remarkable considering he's got seven goals in [his last] seven games. I'm pleased with that and I think his fitness will improve even more.
"He's a good passer of the ball with good vision in his passing, too. The goal he made last week for [Shinji] Kagawa against Tottenham is evidence of his passing ability."