Stewart, with 10 goals in 20 appearances in all competitions, has been one of the standout performers this season.
But Sunderland’s automatic promotion hopes have suffered a severe dent in recent weeks and under-pressure Johnson has demanded more from his players, including Stewart.
Speaking after the disappointing 1-1 draw away at Shrewsbury on Tuesday night, Johnson – when asked whether the service into him was good enough – says the striker will have to work harder if he is to top the goalscoring charts this season.
Johnson said: "I think he has to liven himself up a little bit at times.
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"I'm the number one member of the Ross Stewart fan club - he could be top scorer in this league, but you have to go out and show the desire to be top scorer in this league.
"In games like this, there was the potential to score one, score two, but I thought his spatial awareness wasn't as good as it normally is.
"We did feed him, particularly in the first half, but his upper body strength and movement is inconsistent at the moment.
"That's his next development phase, where he gets that physicality right.
"He works his socks off - he ran 13.5km against Ipswich on Saturday - and it was a thankless task against a side that had a lot of possession.
"We love him to bits but it doesn't stop me driving his standards, for a player that we believe in and a player that I think has the potential to be the top scorer in the league,” added the Sunderland head coach.
Sunderland head to Cambridge on Saturday afternoon and Johnson is coming under increasing pressure from fans given the teams disappointing run of form over several weeks.
Sunderland are sixth with two games in hand over second-place Wycombe Wanderers but the poor level of performances and amount of points thrown away in recent weeks has raised the pressure considerably.