21-year old attacker McManaman and Japanese winger Miyaichi, on loan from Arsenal, were brought on late in the Latics' 2-1 loss to Fulham on Saturday and their Spanish boss was thrilled with how they performed with more chances likely to come their way.
"That's a real positive for us to have a couple of youngsters of that quality," Martinez said.
"I did think they were influential (against Fulham), the way that they came on and were infectious, they played with a real enjoyment and probably we lacked that rhythm early on in the game.
"So I was really pleased with that and I think it shows the competition we've got now for places.
"It's so important that everyone's got an opportunity. It's great to see Callum's development over the last two or three seasons and I think he showed that he's ready to help the team in many games to come.
"Ryo slowly is adapting and getting ready, and when he's at his best he can be a player that gives you penetration and makes things happen."