The 69-year-old manager, who lost veterans Gary Neville and Paul Scholes last season to retirement, featured youngsters Danny Welbeck, Tom Cleverley, Phil Jones and Jonny Evans in United’s 2-1 away victory over West Brom at the weekend.
“The manager has shown all pre-season he's willing to give the young players a go and he's thrown them into big games,” Smalling told MUTV. “So it's great to see them step up.
"Everyone's really gelling and, with Jonny [Evans] and people coming in like Fabio, it's great to see.
"If you do well, the manager will keep you in the team. It's a big marker to all the other players and the rest of the squad that, if you play well, it doesn't matter how old you are. If you've got that ability, the manager is going to put you in."
Meanwhile, Rio Ferdinand has been ruled out for six weeks with a hamstring injury he suffered at the Hawthorns.