Paul Scholes believes Louis van Gaal made "a mistake" in letting Robin van Persie and Javier Hernandez leave Manchester United and claimed his old club is short of attacking options.

United opened its UEFA Champions League group stage campaign with a 2-1 defeat at PSV in a game marred by a serious injury to Luke Shaw, the England full-back sustaining a double fracture of his right leg after a strong challenge from defender Hector Moreno.

Van Gaal's men enjoyed plenty of possession at the Philips Stadion but Anthony Martial, who started in attack, failed to replicate his debut goal against Liverpool on Sunday (AEST).

"The one thing with the United squad now is that they're short of goals. Short of real top quality centre forwards who guarantee you goals. Van Gaal spoke last year about his squad being unbalanced and I think it's even more unbalanced now without top strikers," Scholes said. 

"They've let three or four strikers go. I think if we'd have kept a Van Persie or a Hernandez it looks quite strong. They can score goals. Martial could be a real threat coming off the bench. I just think that's a mistake."

Memphis Depay got United's goal against his former club but headers from Moreno and, in the second half, Luciano Narsingh completed the turnaround.

With Wayne Rooney sidelined, United only has Martial and youngster James Wilson as recognised strikers. Van Persie left Old Trafford for Fenerbahce over the summer with Hernandez joining Bayer Leverkusen towards the end of the transfer window.