The Bulgarian has scored 15 times this season, with 13 of them coming when he is alongside Rooney.
'Having Wayne back alongside him has definitely helped Berba,' Fletcher told United;s official website.
Eye for goal: Dimitar Berbatov fires home for United at Birmingham last week
'Wayne's been supplying him with some great passes and great assists. This season Dimitar's got himself into the box more and once he's got himself into those scoring positions he's been a lot more ruthless. It's great to see and we hope it continues.
'He and Wayne have a good partnership and we need the two of them scoring goals if we want to achieve success this year.'
United have taken seven points from nine in their festive programme to date. They suffered a blow when Birmingham struck late to deny them victory last week, but rode their luck to claim a vital win at West Brom on Saturday.
Rooney scored his first goal of the season from open play at The Hawthorns, but Fletcher believes Rooney's contribution shouldn't be measured solely by goals.
'Wayne's contribution since he's come back from his latest injury has been fantastic. He's been full of energy, he's been dropping into dangerous areas, putting in some great crosses, playing some great forward passes to Berba.
'Even if he's not scoring goals himself he's still doing a great job and he's an important part of the team.'
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