Sanchez joined United in a swap deal from Arsenal earlier this year, but has since struggled to live up to expectations and was once again singled out for criticism following his display in Saturday's FA Cup final defeat to Chelsea.
"[Sanchez's] performances, well they have to improve. They can't get any worse to be honest with you. I think the next few games of next season are vital to him, he needs to get fans believing again and believing they are going to get close to City. They need a big player," Scholes told BT Sport.
"Paul Pogba does not win you games on his own. Hazard does. Alexis Sanchez doesn't, he's shown that since January, they thought he could, but what we've seen...
"They need two magical players. I think they've got really good players, they haven't got a Hazard, [Cristiano] Ronaldo, [Lionel] Messi, and I know many teams haven't, but the difference between the two was the quality in Hazard, we saw that in big games. United don't have that."
Sanchez has scored just three goals in his 18 appearances since joining United.