The Brazilian forward was forced off after just 19 minutes of the 3-0 defeat. Having minutes earlier clattered Kaoru Mitoma, Martinelli was on the receiving end of a tough tackle from behind by Moises Caicedo.
Caicedo avoided a yellow card and Martinelli tried to continue, but was soon replaced by Leandro Trossard.
Following the game, which saw second-half goals by Julio Enciso, Deniz Undav and Pervis Estupinan all but end Arsenal's title hopes, Martinelli was wearing a protective boot covering his left ankle.
Speaking to reporters after the game, Arteta said on the extent of Martinelli’s injury: “We don’t know. He’s in a boot at the moment. He was very uncomfortable, and he could not carry on playing. We need to scan him and see the extent of that injury.”
Such a measure can be purely precautionary but Martinelli will undergo further assessments over the coming days.
The squad will now have a day off and return to their London Colney training base on Tuesday.
Arsenal are next in action against Nottingham Forest as they look to pick themselves up after a demoralising day.