Ajax were hammered 5-1 on Wednesday night, with skipper Kenneth Taylor sent off early on.
Heitinga later said: "This is one of the better teams, not only in the Premier League but I think also in the Champions League. It was a tough one for us to take, but it is how it is.
"The game plan worked and I think it was hard for them to break the lines. We even had one offensive transition with Mika Godts. It was stable.
"I think they scored two goals from the penalty spot, then two goals were deflected. I cannot blame the players, they were working really hard. We had to make a sub, as we wanted to play with a double pivot and still keep the speed in our wingers and a target man in Wout Weghorst."
On the red card, Heitinga added: "We came back into the game, it was 2-1. Then you have to hope that you can bring it into half-time and then go again, but we conceded the fifth goal. The only thing that I have say to the players is that I have a lot of respect for the willingness that they showed on the pitch, working for each other."
He added, "We need to be more stable. It's still going up and down, which is a little bit where the club is at the moment. We're working really hard every day and you can see the improvement. It was only 15 minutes but you could see the improvements already there. Now we have to take the good things into our next match and the things we can do better, we have to learn from that."