Wayne Rooney's injury is a blessing – he needs to be in the gym, says Sir Alex Ferguson

Wayne Rooney is out for a month – time he needs to spend getting fit (Getty Images)

Wayne Rooney's injury may be a blessing in disguise because it will give the Manchester United striker time to get himself properly fit, his manager said yesterday.

Rooney has attacked stories that there is a fresh and growing rift between himself and the club as "absolute rubbish". Nevertheless, Sir Alex Ferguson is known to have been concerned about his levels of fitness since he returned to Carrington for pre- season training after a three-week break mostly spent in California following two very average performances in the European Championship.

Ferguson said the reason Rooney was dropped to the bench for last Saturday's win over Fulham was that he was not fit enough to start. The deep wound he suffered in a collision with Hugo Rodallega's studs will keep him out for another month, time his manager expects to be spent in the gym.

"Rooney wasn't fit," said Ferguson. "He said himself that he needed a few games and I wanted to play Robin van Persie from the start at Old Trafford.

"I knew he [Van Persie] wasn't fit either. In fact, it was only because we lost the second goal that I was forced to keep Van Persie on. I couldn't really start the two of them and I wanted to start with Van Persie.

"Wayne is not as advanced fitness-wise as Shinji Kagawa or the other midfield players. It wasn't an easy choice because you are leaving out a player who can get you goals. It wasn't an easy decision but it had to be done. The injury is maybe a blessing because he can concentrate on his fitness – in the gymnasium in particular."

Rooney turns 27 in October – the kind of age where long-term fitness starts to matter – and he falls into the category of a wonderful ball player rather than a natural athlete.

Rooney's reservoir of energy has always been immense but more than ever Manchester United's game is based on pace; something that shunted Dimitar Berbatov towards the exit at Old Trafford.

The Bulgarian will be ranked alongside Juan Sebastian Veron as a technically gifted footballer who never quite justified the enormous transfer fee Manchester United paid for him. Only in the first half of the 2010-11 season did he really give full vent to his abilities but thereafter he became less and less the focus of United's attack, supplanted first by Javier Hernandez and then by Danny Welbeck.

"He is a very, very talented player," said Ferguson of Berbatov. "His contribution in the home games was his strongest point. He is the only player to have scored a hat-trick for us against Liverpool.

"But there came a time when we changed our game and that didn't suit him because we started playing with more speed.

"Teams were getting in quickly to organise against us so therefore we had to change the way we wanted to play and that didn't suit Dimitar at all."