The Sun says the Bulgarian striker has been struggling for weeks and was not helped when he got a knock in the recent win over Blackburn.
United sources have now revealed Berba has fluid on a knee.
The trouble kept him out of the defeat at Chelsea.
But club medics hope they can manage the problem so he can avoid going under the knife. Yet an Old Trafford insider admitted: "It is causing quite a bit of discomfort, which is worrying."