Smalling has impressed for United this season. The United defender was handed a new four-year contract in April.
“Wayne Rooney is the captain and so you don’t have worry and then (vice-captain) Michael Carrick, but when they are gone, then Chris Smalling can do that, so I push him also in that situation,” Van Gaal said.
Van Gaal admits he has been impressed by what he has seen from the 25-year-old this season.
“He has done it by himself,” he added. “There is a lot of advice. It is about how we defend as a team. He is a part of that and I wanted him to speak.
“He also has the talent to speak. That makes him easy to coach. Because of that, I made him a leader of the team.
“He has made steps and also in a tactical way. When you give instruction in terms of organisation, you need someone to understand that.
“David de Gea has the best view and then the central defenders, so I demand of my defenders that they speak.”