Sporting won 3-0 on Tuesday, with 14 year-old Dowman impressing for the Gunners.
However, Porter later accused the opposition of targeting the attacking midfielder as he was growing into the game.
He told arsenal.com: “Max’s bravery and the way he wants the ball is similar to what Ethan (Nwaneri) is doing with the first team at the moment, in terms of the players trusting him to give him the ball and make something happen.
“I just felt that the referee today… I’m not usually one for talking about referees and it has no bearing on our performance or result, but I actually felt towards the end he put the players at risk in terms of protecting them. He let a lot go which meant there was no consequence.
"The Sporting players then began to target Max particularly because of his talent. That was really disappointing and I communicated that to the ref and his assistants at the end of the game.”
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