Romelu Lukaku did not train with the rest of the Belgium squad on Monday because of an ankle injury, sources have told ESPN FC.
Lukaku reported to the Belgian Football Centre in Brussels as normal on Monday afternoon ahead of World Cup qualifiers against Bosnia and Cyprus, despite being involved in ongoing court proceedings in Los Angeles.
However, the 24-year-old did not train with his teammates on Monday evening as was instead checked over by doctors.
Marouane Fellaini was also absent. He missed last week's trip to CSKA Moscow with an ankle problem but was fit enough to start against Crystal Palace on Saturday.
Lukaku suffered an injury in training with United on Friday, sources said, and he was a doubt to face Palace.
But after passing a fitness test on Saturday morning the striker completed 90 minutes against Roy Hodgson's side, scoring in the 4-0 win. It was Lukaku's 11th goal in 10 games since his summer move from Everton.
Belgium officials remain hopeful Lukaku will be available to face Bosnia in Sarajevo on Saturday and Cyprus in Brussels on Tuesday. Roberto Martinez's side sit top of Group H and have already qualified for next summer's World Cup in Russia.
After the international break, United face a critical run of fixtures with Premier League clashes with Liverpool, Tottenham and Chelsea as well as a Champions League double header against Benfica. Jose Mourinho is already without Paul Pogba, who has missed the last five games with a hamstring injury.
Meanwhile, Lukaku's lawyer, Robert Humphreys, on Monday entered a plea of not guilty on his behalf after the citation for an excessive noise complaint his client received in Los Angeles in July.
Commissioner Jane Godfrey, overseeing the case, ordered Lukaku to appear at Los Angeles Airport Courthouse on Nov. 21. Lukaku can opt not to appear in person at the hearing and can instead have a lawyer handle the misdemeanour citation on his behalf.
After the hearing, city attorney James Eckart said the striker faces both a fine and an additional sum to cover the cost of the police call-outs that occurred over five days.