Atletico won 3-2, with Alvaro Morata scoring twice.
Simeone said: "It was a very difficult game. We knew that we were up against a great opponent, a side who are fast, dangerous and full of a lot of young players. And that's exactly what we came up against.
"We took advantage of the few chances we had and could have also scored one more with Molina in the first half. In the second half we were good for the first 20 minutes, better. The goal gave us more energy and we pushed the game for a bit. But then we started to fatigue and it's hard. Oblak made some great saves and we had to defend. It was a difficult game until the end.
"It shows that this team has weapons, the tools to come back, to attack. But we have to improve in other aspects. We are playing every three days and we have very few players because of injuries and it's difficult. But it is what it is. You have to play well, you have to run and win."
On Morata, he added: "He is very good, and has a lot of desire. He gives the team what he can. The other day it was Correa and today it is Morata. I'm very happy with that."