Robert Snodgrass' omission from the West Brom squad beaten at West Ham on Tuesday is being investigated by the Premier League.
But he missed the West Ham game because of "an agreement between the clubs", West Brom boss Sam Allardyce said.
The Premier League will examine whether competition rules have been broken.
Under normal circumstances, Snodgrass would have been eligible to play against West Ham, as his transfer was permanent.
It is only with loan deals that such restrictions apply in the Premier League - in those circumstances, a player cannot face their parent club.