The winger spent the second half of last season at Hillsborough and played a key role in helping them achieve automatic promotion.
But he played against the Owls in Tuesday's 2-0 pre-season defeat in Albufeira and will not be returning unless they table a bid in excess of £700,000.
"The ball is in their court. We are in no rush to sell Michail and there are plenty of clubs who are interested in him," Hammond told the Bracknell Forest Standard.
"We're having a good look at him over these 10 days in Portugal and at the moment he is very much part of our squad."
Antonio has just one year left on his contract.