Martinelli had not played since the victory over Everton in late August having missed the last five games with a hamstring injury.
The Brazil international was not expected to feature again until after this month’s international break but was a surprise inclusion in the squad for the visit of the Treble winners on Sunday.
Martinelli was introduced at the start of the second-half, replacing Leandro Trossard who had picked up a knock in the first period.
The 22-year-old gave his side fresh impetuous going forward and scored the game’s winning goal four minutes from full-time, with his strike taking a wicked deflection off Nathan Ake to beat Ederson in the City goal.
Speaking at full-time, Arteta explained the club’s physios were reluctant for Martinelli to be involved ahead of schedule, with the player himself pushing to be included in the squad.
‘Yeah it was great,’ Arteta told BBC Sport when asked about Martinelli’s impact.
‘First of all thank you because he has made a huge effort to be in this game, nobody believed that he was going to be available. He kept saying “I’m going to be there, I’m going to be there.”
‘All the medical staff was trembling and he was there when he was needed because Leo got injured after 30 minutes and he was excellent.’
In his post-match press conference, Arteta added: ‘He’s been saying for a few weeks ‘I’m going to be there against City’.
‘Everyone was saying Gabi it will be too early for you. Yesterday he said “boss, I told you I’m going to be ready for this”.
‘It happened that Leo, after 30 minutes, felt something in his hamstring. I turned around and [Martinelli] was ready to go with his kit. “Boss, I’m ready”.
‘It’s the joy of a kid. His mentality is incredible at his age. It’s a joy to have him. Today he could change a game and he was really helpful for us to win the game.’
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