The United centre-back said: "Robin is an important player.
"The manager saw something in him and made it his mission to buy him this summer.
"At the moment, thankfully, he is doing what he came here to do.
"He is not only scoring goals. He holds the ball up well and brings others into play. He does a fantastic job.
"But we are not at the end of the season yet. No medals have been handed out.
"We don't want to jump about too much. Up to now he has done fantastically well. But the job is still there to be done."
Ferdinand praised his United side, he added: "Winning the league is down to a group of players rather than one individual," said the former England star.
"But it is easy for people to settle in here.
"We have a great bunch of players and there is a great ambience in the squad.
"It is a great place to come to if you are willing to come in, work hard and be a member of the team."