The pair have impressed in their appearances so far this season.
Phelan told manutd.com : "Alex [Büttner] is an up-and-coming young player who has done well but is still finding his feet and Nick [Powell] came on against Wigan and scored a great goal. He'll want to keep impressing and be part of this team.
"This is what being at Manchester United is all about. You're preparing, training, playing and recovering for the challenges that are coming your way every three or four days and the whole squad now comes into play."