The Chile international moved to the Red Devils in a swap deal from Arsenal in the January transfer window.
Sanchez opted to move to Manchester United ahead of interest from Premier League leaders Manchester City.
The South American has only scored one goal in 11 games in all competitions since his switch to Old Trafford from Arsenal.
Sanchez has faced criticism over his lacklustre performances in the Manchester United team.
The 29-year-old was unable to prevent Manchester United from suffering a shock 2-1 loss to Sevilla in the Champions League last-16 clash.
Former Arsenal midfielder Merson is adamant that he isn’t surprised that Sanchez is struggling at the Old Trafford outfit.
“[Man United have progressed this season] in terms of points they have, but in terms of football they haven’t,” Merson told Sky Sports.
“Bringing Alexis hasn’t worked – I didn’t think it would work if I’m honest. “
Manchester United are in second place in the Premier League table and 16 points behind runaway leaders Manchester City.
The Red Devils have signed eight players since Mourinho took over the reins at Old Trafford from Louis van Gaal in 2016.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news