The Dutch champions kept their hopes of reaching the knockout stages alive with a 1-0 win over Celtic in Amsterdam last night, which puts them one point behind second-placed AC Milan with two games remaining.
De Boer admitted that his side lost their focus after taking the lead against Neil Lennon's men, but believes that the right team prevailed in the end.
"After we went 1-0 up, I did see us relaxing a bit while Celtic became more of a threat," the 43-year-old told reporters. "Some guys were very tired, but we had a chance to score more.
"I have said before that if you play like this in big parts of a match – and you analyse it – then usually the best team wins. Today the best team won.
"Every team still wants to play for second place and it's all wide open. We have to go to San Siro and we have to play with confidence there, but first we have to play against Barcelona. Any point we can take there is a bonus because we all know how it is to play against them."
Ajax host Barcelona on November 26.