United boss Jose Mourinho is understood to be on the move to a new central defender and was left disappointed when the club failed to get him one in the summer.
He has been linked to a move for Toby Alderweireld as he could be available in January because of a clause in his contract.
Alderweireld will be available for £ 24.5million in July – a huge reduction on the £ 60m they were said to be when United were interested last summer.
However, Berbatov urged United to flex their financial center – even if that means bringing center to back to help Mourinho's struggling side.
"They need to reinforce the back especially center backs, one or two," he told the Mirror.
"In every position they have several quality players and everything they need, so just defense needs reinforcing.
"Hopefully they can find one or two and then they should be okay.
"It's more difficult in the winter, the transfer window then the summer one to make signings for Mourinho, but it should be able to find one person he wants to bring in.
"So if there is someone he wants to bring in, he should listen to him because it's obvious that the club isn't perfect right now and it's in the position that they want to be in.
"They need to improve on one or two people, it shouldn't be a problem.
"I mean it's not like Manchester United are short of money."
United are currently eighth in the Premier League – seven points off the Champions League places and four behind leaders and rivals Manchester City.
Mourinho will reportedly give a £ 100million transfer budget to turn and turn their fortunes around.
They have been linked with a stunning move for Chelsea 's Nolo Gante, including Harry Maguire and Jerome Boateng.
They are also said to be monitoring Brazil's Everton Soares forward, who recently hinted he might be on a move to Old Trafford.