Celta Vigo coach Paco Herrera admits they could lose striker Iago Aspas this month.
Chelsea and Reading are among those said to be keen, with the Premier League pair in the market for new recruits.
Herrera said: "It seems that there is the possibility of a club emerging that are willing to pay the clause.
"In that case, if the player wants to go, Celta will not be able to do anything."