The hosting Merseysiders dominated much of the contest at Anfield, but they were unable to find a way beyond United's Spanish keeper, who made four saves over the course of 90 minutes.
Speaking to reporters after the final whistle, Smalling said of De Gea: "He pulled off great saves and his distribution, especially with those balls to Wazza (Wayne Rooney) and Jesse [Lingard] further forward, was terrific. That set us up on the attack.
"He is a good shot-stopper and a good organiser too. He is a very calm guy but on the pitch, he is very much talking to us as a unit and we know his game inside out. It's good to know that if we are beaten as a defence, we still have a great chance to keep the ball out."
From his 18 Premier League outings this term, De Gea has kept eight clean sheets.