The young striker is clearly still growing - both on and off the pitch - since breaking into the Red Devils' first-team squad 18 months ago as an unknown 17-year-old.
Rashford was forced to spend large periods of last season on the substitutes' bench as veteran striker Zlatan Ibrahimovic hit the ground running during his debut season at the Theatre of Dreams.
The England starlet, however, is understood to have spent a large part of that time working on his strength in the gym, under the watchful eye of the club's conditioning team.
Mourinho has been slowly won over by the 19-year-old's attitude and is convinced he now has the right mentality to handle the pressure of playing at a club as big as United.
United insiders insist they won't allow Rashford to bulk-up his frame too much as they don't want it to directly impact his explosive speed.