The two clubs are showing an interest, although Middlesbrough are still the only side to have made a firm bid for the striker.
The 28-year-old is still in Australia visiting his father Joe, who is undergoing treatment in hospital for a heart condition.
"We'll sit down and look at what is on the table when he returns to England," said Viduka's agent Chris Page.
However, Valencia are unlikely to make any offer until the future of their coach, Rafael Benitez, is known.
