Mainoo could face the Dane this week in England's second Euros game.
"I have learned a lot from him, watching him last season when I wasn't playing as much, trying to learn and pick up from his game as well," said Mainoo.
"The great journey he has been on since the last Euros is amazing. To be around him so close, he's such a great player.
"If I'm to play on Thursday I'll have to put that to the side to compete against him and hopefully get the win."
United striker Rasmus Hojlund is also in line to start for Denmark.
"I've not spoken to him recently, but I'll probably leave that for after the game," Mainoo added.
"I haven't played against him before. He's a really good friend, it's good to have those relationships on the pitch and you build them off the pitch and around the training ground."