Manchester - Manchester United players Michael Carrick, Ander Herrera, Antonio Valencia and Marcos Rojo missed training on Tuesday on the eve of their side's Champions League game against Wolfsburg.
Defender Rojo has missed United's last two games with a hamstring problem, but Carrick and Valencia started Saturday's 3-0 win over Sunderland in the Premier League, with Herrera an unused substitute.
The quartet's absence meant that youth-team players Josh Harrop, Donald Love and James Weir were drafted in for the session at United's Carrington training base southwest of Manchester.
United manager Louis van Gaal is expected to provide an update on his squad's injuries when he speaks to the media later on Tuesday.
Having lost 2-1 away to PSV Eindhoven in their opening Group B match two weeks ago, United will be looking for their first win of the campaign against Wolfsburg at Old Trafford on Wednesday.