"I persistently waited for the chance of the past months," Mitrovic told Serbian website Zurnal.
"I'm sorry after all, I loved Newcastle, but it's time for me to look more at myself, to find the best solution for my career.
"I tried to be honest with myself and my team-mates at every training and match. The fact that I did not get a chance is simply not up to me. Everyone knows that I was left in the background.
"I never felt better, I'm physically in flawless form, I cannot wait to play. The most important thing is to find a club and finally get a chance in the field."