The Red Devils were 1-0 down at the time of De Gea's spot kick heroics against Aleksandar Mitrovic.
Post-game, Ten Hag said: “I think it shows even at that age, you can progress. It was a great save.
“A real step up in his performance and I think that it's similar for our team during the season, good is not good enough and you have to do better.
“It was a great save in the right moment in the game because, 2-0 down, then it's going to be difficult to win the game.
“From there on, we turned the game, and we win the game, so yeah, I'm happy with the performance from today from my team."
He added: “I think after the save of David, it was the turning point and I think we woke up, so a very good wake-up from David, a great save."