Powell scored both goals in Wednesday's win against West Ham with Januzaj playing just behind him during the 2-0 triumph.
"Adnan is a beautifully balanced player," Sir Alex told Inside United. "He's only 18 and still to grow into his frame. But he has good balance, good acceleration and he's a very good technical player.
"He and Nick, of the younger players, are at the top. They're very good.
"Nick has had a few injuries and has had a bit of bad luck so we feel Carrington is the best place for him at the moment [rather than sending him out on loan]. That way, we can work on bringing him on physically and preparing him for next season."
Meanwhile, Reserves coach Warren Joyce has also spoken about the talented teenagers recently.
"Nick's got talent," he told ManUtd.com. "He can play anywhere on a football pitch as he's got really good ability.
"Adnan is still developing but there's no doubt he's got talent too. He's got a good footballing brain and a good touch. We're trying to get him stronger and fitter and he's getting to know the Manchester United style of play as well.
"He's grown up a certain way and I think he has had a tremendous education over in Belgium. He's come over here with good technique and is desperate to be a footballer as well so he's got a lot of things going for him."