The Serbian sat out the first four matches of the league season due to suspension, was then forced off just half-an-hour into the EFL Cup tie with Cheltenham Town with injury, missed the victory over Brighton and Hove Albion due to concussion and was an unused substitute at Derby County.
However, Rafa Benitez finally gave Mitrovic his Championship bow on Tuesday night, and the Serbian responded by scoring during the Magpies’ 6-0 hammering of Queens Park Rangers at Loftus Road.
“It has been a really frustrating few weeks for me, and it has been hard for me to keep my head right,” Mitrovic admitted.
“But I always knew I would come through OK and the game against QPR proves that.
“It has been hard for me to stay strong in my head, but I knew I just had to wait for my chance and everything would be OK.”
Mitrovic did also make an appearance for Serbia earlier this month, but was withdrawn after an hour due to his lack of match fitness.
And Mitrovic - who turns 22 on Friday - accepts it will take him a few games before he really gets up to match sharpness.
“I think you could see from the game that I am still not 100 per cent there,” he added.
“Physically, my fitness is still not quite at the best level, where I want it to be.
“I still need a couple more games to get my best level back. I will get there though.
“Some guys have played six or seven games, this was just my second game of the season and by the end, it was getting harder and harder to run.
“I just need to keep working hard, and then every game will be better and better.”
Dwight Gayle, who has four goals already this season, was left on the bench at Loftus Road - while Ayoze Perez played and scored.
What’s more, summer-signing Daryl Murphy is yet to make an appearance for the Magpies, proving just how fierce competition for places at United is right now.
But Mitrovic is adamant this will bring out the very best in his game.
“I feel like I can definitely score goals at this level,” he continued.
“I know I still have to improve, particularly with my fitness, but I am confident I can score goals this season and help Newcastle win games.
“It obviously helps because I have a lot of good players around me, and that’s important for everybody.
“There is good competition up front as well, which is good.
“There’s Dwight Gayle, Ayoze and now Daryl Murphy as well. I like that.
“We will push each other and get better, which can only be good.”