Manchester United captain Nemanja Vidic admits it will take at least two months before he is back to his best.
Vidic completed the second game of his comeback from a knee operation in Saturday's 2-0 win over West Brom at Old Trafford.
He played a big part in helping United's back-four achieve only their fourth clean sheet in the Premier League this season, maintaining their seven-point advantage over Manchester City.
But after virtually 16 months on the injury sidelines after rupturing his cruciate ligament during the devastating defeat to Basle last December, Vidic says he is not yet back to his towering best.
"The doctor is managing the games I am playing at the moment," he told MUTV.
"Over the first two or three months I have to really look at that because sometimes I might have a reaction.
"This is a process I have to go through. I am glad I have managed to play after three months. It is nice to be on the pitch and playing games.
"Hopefully I can play more often so I can build my form and play better."
Vidic's next challenge will be Wigan on New Year's Day, though he may have some different partners in the United back-four.
Rafael, who has not featured since the Manchester derby win earlier this month because of a hamstring strain, should be fit to face the Latics.
And Phil Jones should also be available after missing the last two games because of a knock he picked up at Swansea.
"Phil could be ready and Rafa will be ready," said Ferguson.
"We rested Robin [van Persie], Chicharito, Ryan [Giggs] and Paul [Scholes] against West Brom so we've got enough freshness to bring in."