Martinelli, 18, headed in superbly to give last year’s runners-up Arsenal the lead on 14 minutes at the Emirates Stadium before adding a quick-fire second barely two minutes later.
The Brazilian was making just his second start for the Gunners since joining the club in July. He also struck twice on his full debut against Nottingham Forest in the League Cup.
Joe Willock fired in a third for a youthful Arsenal on 22 minutes and Martinelli set up Dani Ceballos on the hour to wrap up another comfortable win in Europe as Unai Emery’s side moved on to six points at the top of Group F.
“The team had a great performance. It’s a very important win for us,” defender Hector Bellerin told BT Sport, praising the dedication shown by Martinelli.
“He keeps his head down, he works really hard, he is doing what the coach wants him to do and it shows on the pitch. Everyone is really pleased for him.”
Arsenal are three points clear of both Standard and Eintracht Frankfurt, who beat Vitoria Guimaraes 1-0 in Portugal.
Ole Gunnar Solskjaer’s United again struggled in attack and failed to muster a single shot on target with the club still searching for a first away win since March.
United are winless in 10 games away from home — a run that has seen them score just five times.
“The conditions were tough but I thought we did very well. It puts us in a good position,” said Solskjaer.
“If you get your draws away from home and win the home games, you’ll be fine.”
Academy product Brandon Williams was handed his first start for United, who saw teenager Mason Greenwood denied by a superb block from Ron Vlaar on 30 minutes.
The game was played in The Hague, on an artificial surface described as “one of the worst” by Solskjaer, after a storm damaged Alkmaar’s home ground in August.
– Stonewall penalty –
United rarely threatened with substitute Jesse Lingard firing wide although Solskjaer reacted in disbelief when Marcus Rashford appeared to be chopped down in the penalty area.
“It’s a penalty 101 times out of 100,” said the Norwegian.
Owen Wijndal and Oussama Idrissi forced David de Gea into a couple of smart saves late on as the draw left United level on four points in Group L with Partizan Belgrade, who won 2-1 away to Astana.
Steven Gerrard’s Rangers conceded an injury-time goal in a 2-1 loss at Swiss side Young Boys.
Colombian forward Alfredo Morelos put Rangers ahead just before half-time, but Young Boys levelled through Roger Assale and Christian Fassnacht grabbed a 93rd-minute winner.
Feyenoord claimed a 2-0 home victory over Porto in a clash of former European champions in the other match in Group G.
Celtic gained a degree of revenge on CFR Cluj, the Romanians who ousted them in the third qualifying round of the Champions League, as Odsonne Edouard and Mohamed Elyounoussi scored in a 2-0 win in Glasgow.
Wolves, in their first European campaign since 1980, rebounded from an opening loss to Braga as Willy Boly’s stoppage-time goal earned them a 1-0 win at Besiktas.
“It’s huge for us. We don’t want to make up the numbers, we want to compete to win,” said Wolves captain Conor Coady.
Espanyol forward Wu Lei became the first Chinese man to score in a major European competition as his side beat CSKA Moscow 2-0.
Austrian debutants Wolfsberg drew 1-1 at home to Roma in Group J, while Javier Hernandez grabbed the only goal as five-time champions Sevilla overcame APOEL of Cyprus.
Azerbaijani club Qarabag beat Dudelange 4-1 in a game that was interrupted in the first half when a drone carrying a flag from the disputed separatist region of Nagorno-Karabakh flew over the pitch in Luxembourg.
PSV Eindhoven defeated Rosenborg 4-1, while Lazio came from behind to beat Rennes 2-1.
