Sanchez joined United from Arsenal in January as part of the deal that saw Henrikh Mkhitaryan make the switch to the Emirates Stadium.
The Chile international has only scored three times in 21 appearances for the 20-time English champions, though, and is yet to register during the 2018-19 season.
Robson, however, has said that Sanchez "is a top-class player" and has backed the former Barcelona attacker to get back "to the top of his game" in the coming months.
"Sanchez is a top-class player. But we haven't seen the best of him at Manchester United so far," Goal.com quotes Robson as saying. "But I'm sure he'll get to the top of his game. This summer was the first summer he's had off for several years, and that should do him good.
"Sometimes, though, when you have a free summer and a fair few weeks off, it can take a little bit more time to come back really sharp and to get into the season.
"These first few games of the season are maybe just what he needs to sharpen him up and then hopefully we'll see the best of him for the rest of the season."
United are back in action after the international break with a trip to in-form Watford.