Roberto Martinez has revealed Barcelona could activate a buy-back clause in Gerard Deulofeu's contract at the end of the season.
The 21-year-old winger spent the 2013/14 season on loan at Goodison Park and played for Sevilla on another temporary deal last season, before returning to Merseyside on a permanent basis in the summer.
He has impressed since his switch back to England, providing six assists in 10 Premier League outings so far.
Everton are in danger of losing Deulofeu without being able to block any potential departure, however, thanks to the terms of the deal.
Barcelona, who are subject to a transfer embargo until January, refused to let Deulofeu leave the Nou Camp without permitting a buy-back clause valid for two years.
They can recruit Deulofeu once again for a fee of around £6.3m in the summer then about £8.4m in either of the transfer windows next season.
Everton would then receive a percentage of any fee should he be sold on by the Catalan side once again in the future.
Martinez said: "There is nothing we can do if they activate the clause. That was the deal and the only way we could access the player in the terms that we wanted. I'm delighted with the deal.
"There are certain things in the deal. It is 9m euros for the first year and 12m euros in the second year.
"They can't take him back in the winter so they can't take him back in January. My worry would be to lose him in January. That would be a big problem.
"But for us to have a player of that quality for the money we paid, if we lose him it will be on a profit financially so we would be able to use the money.
"You're talking about a Barcelona player who has been all through his years as the golden boy believing he is going to be at the top level at Barcelona and we got him for a very good price.
"You want the player. I would like to tell Barcelona that he's rubbish but they come to watch our games."