The French star suffered the disappointment during Tuesday's comfortable 3-0 Champions League victory over Swiss champions FC Basel, which the Red Devils fear could now sideline him for six weeks.
United boss Jose Mourinho will deliver an update later today after the 24-year-old underwent a second scan at Carrington on Thursday.
There are fears Pogba may have also suffered a thigh strain after stretching to make the challenge and United's medics have not been painting a positive picture.
Regardless of the diagnosis, United sources state the France international will be ordered to rest for a fortnight and catch some sun prior to him beginning his rehab.
The former Juventus star had started this season in sparkling form, scoring twice, to help his club to the top of the early Premier League standings.