LONDON (Reuters) - Manchester United manager Louis van Gaal says his expensively assembled squad have not challenged for the Premier League title this season because they do not have a prolific striker like their rivals.

He has come under scrutiny for implementing a number of formations and playing forward Wayne Rooney in central midfield, with his team in fourth.

The Dutchman claims the club are 13 points adrift of leaders Chelsea and eight behind local rivals Manchester City in second because his strike force have failed to match the exploits of Diego Costa and Sergio Aguero, who have 17 league goals each for their respective teams.

Netherlands captain Robin van Persie is United's top scorer with 10 league goals while Rooney and Radamel Falcao have struck eight and four respectively.

"We cannot deny that at this moment we don't have a striker who scores 20 goals in the season," van Gaal said.

"Robin van Persie cannot deny it, Falcao cannot deny it and Rooney - but he is not playing there so much any more."

Van Gaal sold England striker Danny Welbeck to Arsenal and loaned Mexico's Javier Hernandez to Real Madrid as part of a summer overhaul after taking over at Old Trafford. He has put faith in 19-year-old James Wilson as the fourth striker.