The Red Devils pulled off the biggest transfer of the January window on Thursday when they finally completed the signing of Fernandes in a deal which could rise to £67.7m.
The 25-year-old arrives at Old Trafford with a big reputation having scored 64 goals in 137 games from midfield for Sporting Lisbon.
However, Berbatov has warned against expecting too much from the new arrival and believes that more signings are needed if United are to turn their fortunes around.
"It's a big signing and United have paid a lot of money for Fernandes and I hope he will give the team a boost, but to rely on just one man to turn things around is going to make things difficult. I would like to be proven wrong and of course I want him to do well but it's foolish just to rely on him. It shows intent from United and I hope they buy someone else," Berbatov told Betfair.
"He is joining up with a young, great team, and the players that are there will see that effort is being made to improve the quality of the side, and I'm sure they will want to up their game and impress, especially the midfield players because they have competition now and I'm sure they will be telling themselves that they need to put more effort in during training and matches.
"Overall, it's good because it shows the intent and calibre of Manchester United, I hope other big names see this signing and think that they was to come to Old Trafford too because like I said just signing one player is a good first step but they need a couple more. When someone pays a lot of money for you, you are always going to have that pressure on your back. Pressure from yourself, pressure from the fans, pressure from the press, pressure from everyone.
"That's why it is up to him and the people around him to make things comfortable so that he doesn't think about those pressures, otherwise it's going to be a problem and of course his teammates will help him to settle. Let's not forget that he is coming from outside of England and now he will be in a different league, with a different tempo and different teams.
"Manchester United had been chasing him for a while and hopefully everyone makes him feel great because he needs to step up straight away into the team and help them."