Martinez required a second round of surgery on a metatarsal fracture he sustained against Sevilla in April while Casemiro suffered a hamstring injury against Newcastle United last week.
United confirmed Casemiro would be sidelined for “several weeks” and manager Erik ten Hag indicated the Brazilian would not return before Christmas.
“I have some time schedules, yeah, but it&aposs always difficult to predict that because they can get setbacks during the rehab period so I&aposm always holding back a little bit on some issues,” Ten Hag said.
Read more: Sir Jim Ratcliffe holding virtual meetings with Man United owner Joel Glazer
Read more: Why Marcus Rashford was not in the Manchester United squad vs Fulham
“I expect some back but Casemiro and Martinez are really strong injuries and I don&apost expect them back before Christmas. It&aposs several weeks.”
United&aposs only matches after Christmas are at home to Aston Villa on Boxing Day and away to Nottingham Forest on December 30.
Left-backs Tyrell Malacia and Luke Shaw are expected to be reintegrated into the United squad some time after the November internationals. Malacia had knee surgery in the summer and Shaw has been sidelined since August 19 with a hamstring tear.
Ten Hag confirmed Harry Maguire and Marcus Rashford were both fully fit and available for United&aposs Champions League group stage game against Copenhagen on Wednesday evening.
Maguire required a concussion check during the 1-0 win at Fulham on Saturday and also sustained a hand injury while Rashford failed a fitness test on the morning of the game.