Athletic Bilbao want to bring back four of their former players, with Manchester United midfielder Ander Herrera at the top of their wishlist.
Herrera left the Spanish side for Old Trafford in a £29million deal in the summer of 2014 and has gone on to win the FA Cup, the League Cup and the Europa League.
United are yet to renew the 29-year-old's contract and he will be available for free at the end of the season if he fails to put pen to paper on a new deal.
That has alerted Bilbao, who are also keen on three more of their former players - Tottenham striker Fernando Llorente, Bayern Munich's Javi Martinez and Alaves winger Ibai Gomez.
The La Liga outfit only sign players who are from the Basque Country - an autonomous community in northern Spain - and are therefore limited when it comes to the players they can target.
'We are interested in Javi Martinez, Ibai Gomez, Ander Herrera, Fernando Llorente... we are interested in all of them,' sporting director Rafael Alkorta told Spanish radio station Cadena Ser.
'Any player who can represent Athletic (through their Basque roots) will interest us and players in that quality bracket interest us, so yes.
'We are aware none of these deals would be easy but we are interested in exploring the possibilities of what we can do.
'We have not made any moves yet but we will get to work this week and get going on these fronts.'