Manchester United signed the 29-year-old in a swap deal earlier this week in a transfer that saw Armenia international Henrikh Mkhitaryan head to Arsenal.
Sanchez made his first start for the 20-time English champions against the League Two side in the FA Cup fourth round on Friday night.
The South American wasted no time leaving his mark on the Red Devils team after the former Barcelona winger created two goals in a 4-0 win over Yeovil to secure their place in fifth-round draw.
Sanchez was a regular in the Arsenal team this term despite uncertainty surrounding his future at the north London side.
But the Manchester United boss still wanted to ensure Sanchez got some valuable first-team minutes at the earlier opportunity despite moving to the Old Trafford side a mere three days earlier.
“I think he needed to play,” Mourinho told his post-match media conference following Friday night’s 4-0 win against Yeovil.
“He didn’t play for Arsenal the last two or three games and his training was not good because half of his body was in the Emirates and half in Manchester, so you don’t train well. He needed to train well like he did that and he needed to play.
“I was not sure about 90 minutes, I wanted to play him for 60, 70 or 75 and that was good because people start to understand his game better and he can understand his team-mates too.”
Manchester United will make the trip to Wembley on Wednesday night when Sanchez could line up against his former club’s bitter rivals Tottenham Hotspur.
The Red Devils finished in sixth place in the table last term.
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