Angel Di Maria was ruled out of Manchester United's 3-1 victory over Newcastle with a pelvic problem.
Manager Louis van Gaal revealed Di Maria had suffered the injury in training on Christmas Eve.
The Dutchman, whose side visit Tottenham on 28 December, does not know when his £59.7m record signing will return.
"It is not a muscle injury," he said. "It is more the pelvis does not have the right relationship with the leg."
Van Gaal added: "We will have to wait and see for the analysis."
Di Maria, 26, only returned to the United side at Aston Villa on 20 December after a three-week absence with a hamstring tear.