The former Gunners midfielder did not hold back with his analysis of the Brazilian as the Gunners dropped more points in the title race with a 1-1 draw at Craven Cottage.
Martinelli thought he had set up a late winner for Bukayo Saka only to be denied by VAR.
Instead, the substitute was offside earlier in the move, causing the goal to be ruled out.
Merson labelled the error as 'criminal' during his analysis of the disallowed goal on Sky Sports.
The pundit and Arsenal legend exclaimed: "In football terms it's criminal what Martinelli has done.
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"I can't express enough how, in football terms, you just can't do that. You cannot be offside, you're looking along the line.
"You've got pace, he's electric so he's not someone who's going to try and nick a yard and play right on the edge. He can give someone a yard or two.
"I'd be surprised if he ain't on the end of one in the dressing room when he gets in by the manager, honestly.
"He even knows, you could see it on his face after, he knew you can't do that.
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"I saw (Matheus) Nunes do that yesterday for Manchester City and Kevin De Bruyne went mad.
"You just can't be offside when you're looking along the line. The one thing when you look along that line, do not be offside, it's as simple as that."
Martinelli had been a second-half substitute, replacing Leandro Trossard off the bench in the 60th minute.
His cameo was his 13th league appearance of the season, with the 23-year-old having scored three goals and added two assists this term.
Arsenal were behind at half time with Fulham forward Raul Jimenez expertly fired the ball past David Raya on 11 minutes.
However, the north Londoners hit back when William Saliba tapped the home when he latched onto a Kai Havertz knock down following a corner from the left.
The point reduces the gap to leaders Liverpool to six points, but the Reds have played a game less as their Merseyside derby against Everton, scheduled for Saturday lunchtime, was postponed by Storm Darragh.