The 21-year old Czech Republic international, who scored 22 goals on loan at Watford last season, has become a wanted man this summer but his agent Ondrej Chovanec says the Italian club only want to sell.
"It is correct that Sunderland are interested, but they are interested in a loan at the moment. They don't want to spend that money at the moment," he said.
"Of course it is very interesting. I know (Sunderland boss Paolo) Di Canio from when he was a player because I was the agent of Tomas Repka, who also played for West Ham.
"To work with Paolo Di Canio would be a good thing. I think he will probably end up coaching with one of the very top teams in England.
"The final decision is with Udinese Calcio and the owner Giampaolo Pozzo and there are a lot of clubs interested in this player.
"Matej is a quality player and a top player for them, so they will be looking for the best deal available. He can play in a top level and I think that is what he will be doing next season."