Magennis signed a three-year contract at Rugby Park this summer after being promised a chance to make his mark as a centre-forward.
The 24-year-old made 123 appearances in four years at Aberdeen but most were off the bench and a number of them from the wing or even right-back.
A loan spell at St Mirren last season saw him play 13 times without scoring but Johnston believes the powerful forward can start weighing in and Magennis is eager to add to his one Killie goal when they host Partick Thistle in the Scottish Premiership on Saturday.
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"The best thing about it is I've been playing," the Northern Ireland international said. "I've finally got a manager who has put his trust in me, which is absolutely fantastic.
"That's the conversation that cemented the deal, that I would get my chance and as long as I'm doing the business I will stay in the team.
"Under Craig Brown I played 15 games at right-back. He needed me to play there for the team and that's the way it is, but when I signed here the manager told me I would be playing up front unless we are struggling."
The former Cardiff player added: "At previous clubs, no disrespect, I was never really the person to be playing up front. It was always in for one, out for a couple, but here the gaffer has put his trust in me.
"I feel I'm repaying it in a way even though I have only scored one, against Ross County.
"I'm harsh on myself, I should have scored against Motherwell and Hamilton. But I'm happy to be in those positions to score and it's great to have a manager who has that faith in me and the only way to repay him is to keep doing solid performances.
"The gaffer talks about goals every day in training and the minimum to work the goalkeeper. I know it's going to be a matter of time before they start going in and hopefully they come thick and fast."
Partick defender Dan Seaborne insists he will have no trouble taking on Celtic after lining up against the likes of Michael Owen and Jack Wilshere.
Jags boss Alan Archibald admitted he was gutted to see his side paired alongside Celtic in Wednesday's League Cup quarter-final draw but Seaborne cannot wait.
The former Southampton centre-back was part of a Saints line-up who pushed Manchester United close in a January 2011 FA Cup tie before goals from Owen and Javier Hernandez clinched a 2-1 win.
And he was also part of a Coventry defence that included former Scotland defender Andy Webster which went down 4-0 against Arsenal earlier this year in another FA Cup encounter.
The scoreline was resounding but Seaborne insists the Sky Blues were not disgraced as they took on a team that included Wilshere, Mesut Ozil and Alex Oxlade-Chamberlain at the Emirates.
Having gone head-to-head with some of the Barclays Premier League's biggest names, the 27-year-old is not worried about the threats Celtic will pose.
Seaborne said: "I've never had a great cup run before but I've played some big teams like Manchester United and Arsenal during my career.
"Getting to a quarter-final is just as exciting for me. It's new territory for me being this far down the line so I'm looking forward to it.
"The Man United game was close but we should have got something from it. They had all their big names on show but we just didn't have the experience you need to see out a game like that.
"Against Arsenal, we did okay but lost 4-0. They pretty much had a full-strength side out too.
"But having played in those type of games, I definitely feel it can help me when we go to play Celtic. Everyone wants to prove themselves on that kind of stage. The better the player I am playing against, the more it brings out of me.
"There is no pressure on us so if we can go there and do as well as we can, who knows."
Thistle responded well to last Friday's Scottish Premiership defeat to St Mirren by knocking the Buddies out of the cup when they met again four days later.
But with Archibald's team yet to record successive wins this season, Seaborne is keen to add some consistency to their performances ahead of Saturday's trip to Rugby Park.
He said: "We have had a few results that we are really disappointed with. We feel we should have more points that we do. Hopefully we can set that right on Saturday."
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