The Portuguese midfielder was amongst the goals again on Tuesday, scoring twice to set United on their way to a 4-1 victory over Istanbul Basaksehir.
"Fernandes has impressed me so much and the only weakness I see in him is his defensive side," Berbatov told Betfair.
"He can improve a bit there. He's an attacking midfielder, though, who creates chances, scores goals and up the pitch is where he has his power.
"That's why I think it's good to have someone in the team to cover for him."