Stewart in line to make first Cats appearance against Rovers
year deal on Wearside with the club holding the option of an extra year.
Johnson says Stewart will have to prove himself to supporters but says the club have signed an athletic striker who can make a significant contribution to Sunderland's all-round game.
"He's a good player but he's obviously got to prove that," head coach Johnson said.
"He's got to prove that to fans and rightly so, it should be a heavy shirt to wear because we want quality players.
"His rise is a great story and the challenge for him will be dealing emotionally and psychologically with the size of the club.
"He's a really good runner and a really good mover, he's a handful for any opposition defender, the kind of player who can flick it on and get on the end of it himself.
"He works extremely hard from the front which is a prerequisite for players at his club, certainly in that front four where they have to understand the pressing triggers we work to.
"He's got a bit of improvement [to make] in terms of his finishing from the floor, I think," Johnson added.
"He's a good, crisp passer of the ball, but we need to do a bit of work on lowering his centre of gravity to be able to adjust and finish, like a Kevin Phillips.
"The size of him, he's more of a Quinn than Phillips, but he certainly doesn't play in that mould, he's a runner similar to an Aiden O'Brien.
"He can hold the ball up, he's got a good touch and is tall.
"It's always good when you can sign a tall player because for obvious reasons I'm quite attracted to those silky, little footballers who cam press and be agile, and we get that with a tall player as well."