Barcelona midfielder Matheus Fernandes has picked up a right hamstring strain during training on Monday.
Fernandes then spent the second half of the season on loan at Real Valladolid, but he only played 166 minutes of football during his time with Sergio Gonzalez's side.
There was a chance for the 22-year-old to try and impress Koeman during pre-season training, but he has been unable to do so.
Marca says Barca are seeking a buyer for him.
However, the injury he picked up has thrown a spanner in the works, with his recovery set to determine how long he will be out for.