Grant McCann admits it will be a struggle for new signing Kieran Sadlier to be fit enough to be selected for Doncaster Rovers in early January.
The forward is currently training with Rovers after agreeing an 18-month contract following his departure from Cork City at the end of the League of Ireland season last month.
International transfer regulations mean he remains ineligible to feature for the first team until January 1.
In the meantime McCann had hoped to select Sadlier for U23 fixtures as a trialist in order to build his fitness.
But further regulations have blocked such an approach, meaning Rovers must wait until next month in order to work on the 24-year-old's match sharpness.
"Unfortunately we thought he could play as a trialist but we need to get international clearance for him," McCann said. "Even then, they've come back and said he can't play until January.
"It's a bit of a sickener really for us because it's going to be hard to fit U23 games in now for him going into January.
"We'll try to get as much into him on the training ground and we'll try to maybe have a couple of in-house games against the U18s if we can fit them in to get him up to speed.
"It'll possibly be the early part of January for Kieran now.
"We've got some U23 games in the early part of January - there's a game with Notts County on the 8th.
"It'll probably be a game for him but we'll try and work around it and get him up to speed quicker."
Rovers U23s suffered a 3-2 defeat at Peterborough United on Tuesday. Tommy Rowe, Matty Blair, Alfie May, Marko Marosi and Paul Taylor all featured with Blair and Taylor putting Rovers 2-0 up.