Michael Carrick, Ander Herrera, Antonio Valencia and Marcos Rojo were all missing from the session at the Aon Training Complex in Carrington on Tuesday morning.
Rojo has missed the last two games with a hamstring problem, but Carrick and Valencia started Saturday's 3-0 win over Sunderland while Herrera was an unused substitute.
Louis van Gaal is expected to reveal the details of the injuries at a press conference on Tuesday afternoon.
Youngsters Josh Harrop, Donald Love and James Weir were drafted in for the session because of the injuries.
Defender Paddy McNair also took part after he recovered from the injury that sidelined him for the last two games.
After losing against PSV Eindhoven in their first Group B match earlier this month, Van Gaal will be looking to record his first Champions League win with United against Wolfsburg at Old Trafford on Wednesday night.
Source : PA
Source: PA