The Red Devils signed the Chile international in a swap deal that saw Armenia international Henrikh Mkhitaryan complete a move to Arsenal in the January transfer window.
Sanchez made two assists on his Manchester United debut in a 4-0 win over League Two side Yeovil Town in the FA Cup third round.
The South American has only scored one goal in the Premier League since his switch to the Red Devils from Arsenal.
Sanchez netted Manchester United’s second goal from the penalty spot to open his Premier League account for the Red Devils.
The January signing struggled to leave his mark in Manchester United’s disappointing losses at Tottenham Hotspur and Newcastle United.
Former Liverpool FC captain Redknapp believes Mourinho is struggling to accommodate is wealth of attacking talent in the same team.
“I am sure Jose is scratching his head and wondering how to get the best out of his team,” Redknapp told The Debate on Sky Sports.
“I’ve seen the Tottenham and Newcastle game since Sanchez has been there. They are struggling.
“As much as Sanchez is going to be a great addition, I think Jose will struggle to get all those attacking players in his team because it’s not how Jose likes to set up.”
Manchester United are in second place in the Premier League table but only two points ahead of Liverpool FC in the top-four race.
The Red Devils will take on La Liga giants Sevilla in the first leg of the Champions League last-16 clash on Wednesday night.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news