Diouf was a key performer for Matsatsantsa last term, notably winning the PSL goal of the season award.
However, he last played for the team in a 1-1 draw with Mpumalanga Black Aces in the League on September 15.
"Diouf is doing well at training," Johnson tells KickOff.com.
"There is number one, two, three and if the number one is not doing well, you put in number two to allow the number one to build his reputation at training and work hard to reclaim his position in the team.
"We are not only building 11 players but 29 players in the team.
"So far he is doing well at training and will get his chance."
United progressed to the Telkom Knockout quarter-finals on the weekend with a tense 2-1 victory over Ajax Cape Town.
Johnson feels his men allowed the Urban Warriors back into the game at Lucas Moripe Stadium.
"I believe we did well, only to allow Ajax Cape Town to come back into the game in the second half," he says.
"It was always going to put us under pressure with Ajax chasing the game in the second half.
"I think we slowed down the tempo in the second half and the head injury to Davies Nkausu didn't do us any good," concludes the former Platinum Stars mentor.