Heerenveen will be without Paul Bosvelt's services in the short term due to a heart problem.
The former Manchester City midfielder has been told to rest as he has an inflamed pericardium - the sack that surrounds the heart.
Bosvelt, who is set to retire at the end of the season, will now miss the club's must-win Uefa Cup clash with Lens on Thursday.
"Normally the pericardium is smooth but at the moment it is rough because of the inflammation, which causes a lot of pain and keeps me awake at night," explained Bosvelt.
"My pulse cannot go above 140 which is why I am wearing a heart-rate monitor.
"I can ride a bike to keep me fit until the problem is solved, but I won't force it as I am eager to get some sleep at night."