The Amsterdammers lost ground in the chase on Sunday after a surprise 2-1 defeat to Roda JC and Koster knows they cannot afford to slip up against the likes of Friday's opponents Sparta Rotterdam if they are to secure second spot.

The chances of clawing back PSV's nine-point lead appear slim but the battle to be runners-up is intensifying with Ajax level on 44 points with three other teams - Feyenoord, Heerenveen and NAC Breda.

"It is getting harder and harder," Koster said. "But we have to live from match to match and try to win as many games as possible.

"I think that the way we have played after the winter break has improved a lot. It's only frustrating that we can't always see that on the pitch.

"(Against Sparta) we have to play to our own strengths. That's the most important thing. We have to take the three points, no matter what."

Koster is buoyed by the return of Bruno Silva from a calf injury and captain Edgar Davids from suspension.

Vurnon Anita (hip), and Laurent Delorge (Achilles) are back in training but not yet fit enough for selection, while Nicolae Mitea (leg) and Gregory van der Wiel (knee) remain sidelined.

Sparta coach Foeke Booy has called on his side to bounce back from defeat to basement side and city rivals Excelsior last time out.

"The game against Excelsior was a big disappointment," he said.

"This week we realised we needed to win that much, but it's too late. The game against Ajax will be difficult but my team know that we have to get a result out of it."

Olaf Lindenbergh is suspended for the visitors, with Yuri Rose (foot), Nathan Rutjes (foot), Charles Dissels (muscle) and Iderlindo Moreno Freire (muscle) also unavailable.