Joel Pereira is undergoing medical tests at Manchester United.
The Manchester Evening News says Pereira has returned to Manchester United, temporarily, from his loan spell at Hearts to get injury treatment.
The Portuguese moved on loan to the Scottish club this summer and made his first league appearance in their 3-1 defeat at Celtic on Sunday - with teammate and friend Andreas Pereira in attendance at Celtic Park.
It is thought Pereira's thigh injury is not overly serious, though he wont play for Hearts in their next game against Hamilton.