The youngster made an immediate impact as a second-half sub in the FA Cup tie with Bournemouth, rattling the bar and sparking the fightback that earned a 1-1 draw.
He is set for his Premier League debut at Fulham today.
Martinez said: "The way Angelo put himself about showed what he will bring - and it's something we didn't have in the squad.
"He is very quick and that allows him to stand on the last man and find a yard or two of space. He showed incredible know-how for someone of his age."