The Serbian is currently serving a three-match ban, but was on target for his country during the international break.
The Serbian has had a tough start to life on Tyneside, picking up two yellows and a red card in his first three games for the Magpies, while contributing very little in an attacking sense.
The Toon Army is growing impatient over the striker’s lack of discipline, but Simpson has suggested the 20-year old needs to be given time.
“It’ll be a huge thing (that Mitrovic scored for Serbia) because he’s come in and he’s desperate to do well for us,” he said in the Chronicle.
“He’s so keen and eager and probably that’s been reflected in the cards he’s been picking up, and the red card he picked up in the Arsenal game… we need to control that.
“We want him to have that enthusiasm but we need him just to relax and realise that we brought him here for the long term - we’re not looking for an instant fix.
“We know it’s going to take him time to get settled in and he’ll have got a huge lift getting that goal,” he added.
Newcastle fans will hope Mitrovic comes back from the international break with a fresh outlook and reins in some of the erratic behaviour that has so far dominated his Premier League career.
His dismissal against Arsenal means he will miss the Magpies upcoming games against West Ham United, Watford and Sheffield Wednesday, but could return to face Chelsea at the end of the month.
Should he fail to find the net against the champions, his wait for a first Newcastle goal will drag on until at least October, and further pressure could soon start to mount.