FC Barcelona are currently in the process of waiting for LaLiga to make a decision on whether they will be able to make an emergency signing to cover the absence of Ousmane Dembele, who has been ruled out for up to six months. Once approved, Barça would have 15 days to make a signing who would be available for the remainder of the LaLiga season.
However, should Dembele recover sooner than expected he wouldn't be eligible to feature again in LaLiga. This is part of the rule as the said injured player's registration spot is transferred to the replacement. Nor would Barça be able to sell him, should they so desire, until those five months have been completed.
There is some possible good news for Dembele when it comes to European competitions though. If Barça were to reach the semi-finals and then final, and Dembele was fit again, he'd be available for selection as normal. This is because the French winger had been registered for the competition before his injury occurred.