Wayne Rooney and Fabio da Silva sat out the open 15-minute section of Manchester United's training session ahead of the UEFA Champions League tie with Schalke.
United tackle Schalke at Old Trafford on Wednesday evening and boast a 2-0 advantage from last week's semi-final first leg in Germany.
Rooney scored the Red Devils' second goal in that game, but he did not take part in the opening stages of training on Tuesday morning.
The England striker missed a similar session before last month's quarter-final against Chelsea and is expected to be involved for the visit of Schalke.
However, it is likely that Rooney will begin the game on the bench after Sir Alex Ferguson suggested Dimitar Berbatov and Michael Owen would start the return meeting.
Brazilian full-back Fabio, who has started United's last three games, also did not take part in training after picking up a knock in Sunday's 1-0 defeat at Premier League rivals Arsenal.
Darren Fletcher was involved in training and the midfielder could feature at some stage against Schalke after two months out with a virus.
Denmark keeper Anders Lindegaard has recovered from the knee ligament injury that has ruled him out for a couple of months should Ferguson wish to rest first-choice shot-stopper Edwin van der Sar.