Defender Wes Brown faces four or five weeks out of action after undergoing an exploratory operation on an ankle injury, Ferguson also told reporters.
Berbatov was substituted after 30 minutes with cramp during Bulgaria's 6-1 defeat by neighbours Serbia in a friendly on Wednesday.
"It's not a tear. It's a slight strain but with hamstrings you don't take any chances. There's no point in making it worse," Ferguson said.
"He will certainly be out for tomorrow and he will be out for Tuesday but there's a slight chance he may be available for the City game."
United play derby rivals Manchester City on Sunday week but first face a testing fixture against a Villa side which Ferguson believes are capable of breaking the top-four stranglehold held by United, Chelsea, Arsenal and Liverpool.
Wayne Rooney, who missed England's 2-1 friendly win over Germany on Wednesday after suffering a chest infection, is likely to be fit to offset the loss of Berbatov for the trip to Villa Park.
Rio Ferdinand may miss the game as he struggles to shake off a back injury which kept him out of the Berlin international.
United have won 11 times in 12 visits to Villa Park and Ferguson is keen to build on last Saturday's 5-0 win over Stoke City which lifted them to third in the table.
"We recognise Aston Villa as an improving side and I think (manager) Martin O'Neill has done a fantastic job so we expect a really difficult challenge," said Ferguson.
"We got five goals last week and goal difference could be an issue this year, you never know. This is why we have to push on and start taking our chances regularly because the football we are playing deserves more than we are getting at the moment."
