Mitrovic was signed on loan by Slavisa Jokanovic, who picked him up on loan in January.
He went on to score 12 goals in 17 games as the side were promoted to the Premier League. Then Jokanovic insisted Mitrovic arrive on a permanent deal, which will cost the club up to £27 million.
Mitrovic knows it took courage to invest that type of money in a player who had failed at Newcastle.
"I try to pay Slavisa back because he did everything to bring me here. Sometimes I will score and sometimes I won't, but I give 100 per cent for him," he said to the Daily Mail.