Tigers boss Nigel Pearson is keen to sign the Spaniard who made 12 appearances for Hull during a loan spell last season.
However, the clubs have so far been unable to agree on a valuation.
"I never rule anything in or out but I would say it's unlikely," Pearson said. "Ayala is a target and we've put a bid in. The clubs are in discussion and we'll see where it takes us.
"As always the criteria of how you add to your squad is governed by finances."