Ajax captain Siem de Jong will be out for six weeks, starting with Sunday's match with rivals Feyenoord, after being diagnosed with a collapsed lung.
The 24-year-old midfielder has played in both of the club's Eredivisie matches so far this season.
He had a check-up because of persistent tightness and coughing on Monday, although it is not known how long he has had the problem.
The Netherlands international has spent his entire career at Ajax.
Right-back Ricardo van Rhijn will also miss this weekend's match with Feyenoord after the club accepted a one-match ban following his red card in