The Bulgarian, a £30m signing from Tottenham in September 2008, started the 2-1 defeat by Chelsea last weekend, but it was his first start against either of the other top-three sides.
"There is absolutely no reason why we should doubt that he is a good player," said Ferguson.
"Dimitar has done well in a lot of games recently but we prefer to play with one striker."
Newspaper reports have suggested that a possible deal to supplement United's attack with Valencia striker David Villa failed as United refused to meet the Spanish side's £35m valuation.
But Ferguson refused to confirm any discussions had taken place over the 28-year-old.
"There is conjecture about every good player," he said.
"I am sure by the end of the season there will be half a dozen more. I cannot confirm that speculation but most of it is not right."
A deal to sign 21-year-old Mexico forward Javier Hernandez for an undisclosed fee was announced on Thursday, and Ferguson conceded the age of a player was normally a major factor in deciding his purchases.
"We like doing these kind of deals where we can identify young talent. We have been good at that over the years," he said.
"There is the odd exception when we get a mature player, like Berbatov.
"When you sign a player for that kind of money you know there is not going to be a resale value if he stays with you for six years."
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