De Gea produced three stunning saves in the second half to help United stretch their unbeaten run to eight in all competitions and earn a victory that moved them up to fifth in the Premier League.
Widespread reports in the British media, however, have said that United are not certain to extend the Spanish keeper's contract, which expires at the end of the current campaign.
"This issue about negotiating (contracts), we said first we are focusing on the games," Ten Hag said. "When we come to an end with this block, we have the World Cup and then we have to think about them.
"In the background we have our strategies for how to deal with it. It is clear already and I have already emphasised it several times, I am really happy with David. He is a great goalkeeper.
"He is only 31. He is fit. He can progress even more. He was already impressively good for Man United and I think he will do that in the future as well."
Marcus Rashford headed the winner in the first half at Old Trafford on Sunday, continuing his return to form this term as he passed 100 goals for his club in all competitions.
"When you have scored 100 goals before you turn 25 you are already there," Ten Hag added. "But I think he won't be satisfied just with that.
"It is about how many trophies you win with your club. I think he can develop his game even more. His heading has to improve and he worked on it, and you can see what is happening.
"He is much more consistent and reliable. I am really happy with his progress. He is already a great player, but there is still a lot of room for improvement. He will have a fantastic career I am sure."
