Lukaku missed the FA Cup defeat to Wolves with a foot injury before sitting out Belgium's victories over Russia and Cyprus during the break. Rashford and Martial were named in their national team squads only to withdraw later.
Jesse Lingard was not part of the England squad but spent part of his break training in Dubai and is fit to face Watford.
"We've had a very good training week," United boss Ole Gunnar Solskjaer said. "Some have been away and have come back OK."
United face Watford at Old Trafford in Solskjaer's first game as permanent manager.
With Arsenal not playing until Monday and Tottenham facing Liverpool at Anfield on Sunday, victory over Watford would see United end the day level on points with third-place Spurs.
United are still without Alexis Sanchez, Matteo Darmian and Antonio Valencia. Club captain Valencia has not played for United since Jan. 2 but managed 90 minutes for Ecuador against the United States during the international break.
"Alexis has just come back and there is only him who is out, along with Matteo and Antonio," Solskjaer said. "Those three are out."