Dundee United manager Craig Levein is keen to establish what illness is currently hampering Prince Buaben's fitness.
The normally industrious Ghanaian midfielder lasted just 40 minutes of United's pre-season friendly win over Stirling Albion on Saturday.
It had been feared the 21-year-old contracted malaria while on holiday in Ghana over the summer, although tests have now ruled out that possibility.
Levein is growing ever more concerned at Buaben's predicament and is keen to establish the cause of the illness sooner rather than later.
"Prince has been suffering from a virus and we need to get to the bottom of it," explained Levein.
"He's had his blood taken and nothing major seems to be showing up. He was in Ghana over the summer, so we were a bit worried it might have been malaria or something.
"But he's been checked out and thankfully it's not that. It's a worry, but we'll just have to see how he is this week."