The Amsterdammers travel to Champions League-chasing Heerenveen on Saturday seven points behind PSV Eindhoven with just four games of the season remaining.

Ajax were beaten 2-1 by FC Twente last time out and Koster is still angry his side have all but gifted the league leaders their fourth straight crown.

He said: "We didn't play aggressively and that's something you need to do to win such a difficult match.

"I think it's unbelievable. This is not the way to fight for the national championship. I don't understand how it could have happened.

"We have given the title to PSV, so we have to focus on the play-offs now, hard as that might be."

Ajax should comfortably qualify for the Champions League play-offs, holding a five-point advantage over sixth-placed Twente.

But they could lose second spot to their opponents should they go down to Gertjan Verbeek's side, who claimed a 4-3 success over Roda JC last time out.

That was Heerenveen's third straight victory and lifted them up to third in the table, three points behind Ajax.

And Verbeek has set his sights on a second-placed finish for his side as they look to end the season on a high.

He said: "Second place is not impossible for Heerenveen. A win against Ajax and then wins in the three remaining games will be enough. We have a good chance."

Heerenveen, who this week revealed out-going Gent coach Trond Sollied as their boss for next season, have a selection dilemma in defence.

Long-term injury victim Kristian Bak-Nielsen (ankle) is still out, while Henrico Drost (knee) is also missing and Calvin Jong-A-Pin is a major doubt.

Midfielder Geert-Arend Roorda is out with glandular fever, but striker Gerald Sibon should shake off an ankle knock in time to play.

Gregory van der Wiel (knee), Gabri (hamstring) and Erik Heijblok (shoulder) are Ajax's only injury absentees, while Siem de Jong (groin) is doubtful.