The 22-year old Gambian has spent three years in the system with the Revs, notching up 82 league games, and has improved his game each time but Nicol wants him to step up even further this term.
"This is a very important year for Sainey,'' Nicol told the Boston Globe. "The first year, it was like he's young. The second year, he kind of did some good things. This year, he needs to step up and put it all together, to be honest.
"We've seen some of the things he's capable of, but it has to be consistent. If he does that and keeps doing the stuff we know he can do - that's part of his process, growing up and being the player he wants to be."
Nyassi himself knows that he has to improve his game, especially the mental side, but loved the fact that Nicol was prepared to use young players at all times which in turn will make the Revs a better outfit.
"The difference is, before, we used older guys. But, with Stevie, if you're smart and wise in the game, he doesn't care if you are young or old, he will use you," he said.
"We're getting better, we're getting pace up top, everywhere, and that's where we will improve. And, they're bringing in names these days. I think that will help a lot, in different ways. Hopefully, the fans will come out and that will motivate us more."