The Brazilian youngster was the talk of the Emirates tonight after bagging himself two more goals in just his second Gunners appearance.
Arsenal swept Standard Liege aside 4-0 and it was Martinelli that got the ball rolling in the 13th minute.
The 18-year-old’s well-directed header from Kieran Tierney’s cross broke the deadlock, before he double Arsenal’s lead three minutes later.
Martinelli became a Gunner in a £6m deal this summer but it has since emerged that he could have instead become a Red Devil.
When he was Brazilian club Ituano, the starlet had trials with United as well as La Liga champions Barcelona .
But Arsenal reportedly beat the two European giants, as well as another 15 clubs clubs to his signature.
And Gooners are not likely to let Man Utd fans forget that fact in a hurry.
“Martinelli walks straight into Man Utd’s attack and this is not even banter,” tweeted one fan.
Another fired a broadside, writing: “Man Utd appreciation tweet. Thank you for opting not to sign Martinelli after he had a trial there.
“United: 0 shots on target tonight – Martinelli: 2 goals and an assist tonight.”
“Martinelli is legit better than any striker Man Utd have this is hilarious,” wrote a third.
Arsenal captain Hector Bellerin was asked for his thoughts on Martinelli after his side’s easy win.
“He has been a player, he keeps his head down, he works really hard,” he said
“He is doing what the coach wants him to do and it shows on the pitch. Everyone is really pleased for him.”
Dani Ceballos scored his first goal for the club after Joe Willock’s precise finish had put the home side 3-0 up.
Frankfurt beat Vitoria 1-0 in the other game in Arsenal’s group.