The 18-year-old defender came off the bench at Aldershot, replacing Ezekiel Fryers, to become the fourth player to make his United bow in this season's competition.
Keane also managed to pip his twin brother Will to an appearance as the striker was an unused substitute at the EBB Stadium.
"Michael has had a really good season," enthused Joyce to ManUtd.com. "I can't remember him having a bad training session, let alone a bad game.
"He's a consistent kid who works very hard."
Another of the Reds' recent debutants, Larnell Cole, was also a substitute after getting a brief taste of action at Leeds in the previous round.
"Larnell Cole is another one who I can't remember having a bad session," added Joyce. "He and Michael are really consistent."