The front man signed for Steve McClaren's side from Anderlecht this summer, but hasn't enjoyed the best of starts to his Premier League career.
The fiery forward received a booking less than three second into his debut against Southampton on the opening day of the season.
Mitrovic followed that effort up with another caution against Swansea, despite only featuring for the best part of 15 minutes.
He managed to avoid the referee's scorn during the trip to Old Trafford but hastily undid all of his hard work when he was sent off for a nasty challenge on Arsenal's Francis Coquelin during a 1-0 defeat to the Gunners prior to the international break.
Mitrovic is in the middle of a three-game ban for his tackle on the Frenchman and Newcastle mocked their new signing with a birthday tweet that included a direct dig at the striker's failure to tow the disciplinary line.
Trolled by your own club? Now there's something you don't see every day.