Viduka will cost up to £5m but Valencia might not move for the Aussie until Benitez commits himself to the club.
Liverpool are chasing Benitez as a replacement for Gerard Houllier.
This delay could enable Middlesbrough to seize their chance. Boro can at least offer him UEFA Cup football next season after their League Cup success.
Boro manager Steve McClaren tried to sign Viduka during the January transfer window, only for the striker to commit himself to Leeds' relegation battle.