Man Utd are missing a 20-goal-a-season striker - Van Gaal

Radamel Falcao and Wayne Rooney have scored 12 goals between them in the Premier League

Manchester United have not challenged for the title because they do not have a 20-goal-a-season striker, according to manager Louis van Gaal.

United's top Premier League scorer this season is Robin van Persie with 10.

In contrast, Diego Costa and Sergio Aguero have scored 17 each for Chelsea and Manchester City respectively.

"It is true," said Van Gaal. "Robin van Persie cannot deny it, Falcao cannot deny it and Rooney is not playing there much anymore."

The Old Trafford boss has been criticised for playing striker Wayne Rooney in a deeper position, while Radamel Falcao has had limited opportunities, with Van Gaal questioning the Colombian's fitness.

Van Gaal said: "At this moment, we don't have a striker who scores 20 goals."

United entertain Sunderland on Saturday afternoon, having slipped to fourth in the table following last week's defeat at Swansea.

The result left them 13 points behind Chelsea and eight adrift of City, who have both scored 56 goals overall, compared to United's 44.

In the Red Devils' last Premier League championship-winning season under Sir Alex Ferguson, they scored 86 times, with Van Persie marking his first season at the club by winning the Golden Boot prize with 26 goals.

Van Persie will be missing for at least a fortnight with the ankle injury he picked up at the Liberty Stadium, which Van Gaal says will offer Radamel Falcao, who has scored four league goals since his loan move from Monaco in September, more chance to impress.

And the United manager does not view this season's scoring difficulties as being long-term.

"It doesn't tell you anything about next year," he said.

"They could be having an unlucky year. I have to decide, with my staff, if it is this aspect or others."

Van Gaal intends qualification for next season's Champions League to be the first stage of a three-year process aimed at reclaiming the Premier League.

"If it [the title] came this year I would be very happy. But I don't think it will happen."