Twenty-six-year-old Rooney was criticised by many pundits at the start of the season for appearing to be out of shape.
However, having been out of action for around a month with a leg injury, the England international impressed on his return to the United starting lineup during Tuesday's 2-1 win over CFR Cluj in the Champions League.
Now that Rooney is looking fitter, Ferguson has warned the striker that he must continue to show the same commitment to training.
"Rooney is fit, he has trained really well," Ferguson told reporters. "The period he was injured, I think that was key to it. He maintained his fitness which is important because he is a stocky lad.
"He is very stocky. He is going to have to train well all the time, there is no doubt about that."
Rooney is expected to be included in the United side that is due to take on Newcastle United this afternoon.