Sir Alex Ferguson is an admirer of the Porto star and wants to strengthen his forward line as he bids for a fourth successive Premier League title.
The Brazil international has a clause in his contract that says he cannot be released for less than 100m euros, but sources at the Portuguese club say a realistic offer of around £20m would see Hulk leave the Estadio do Dragao at the end of the season.
"The big clubs in England have been in contact," said the player's agent Teodoro Fonseca.
"They have asked for details about his contract situation but any decision about his future rests mainly with the directors of Porto.
"I am calm about the situation and so is the player because he has an excellent future. I think we could be discussing a move in April or May."
Hulk, 23, whose real name is Givanildo Vieira de Souza, has four years left on his contract and is also rated highly by Arsenal boss Arsene Wenger.
The Frenchman saw him play at the Emirates in last season's Champions League but would almost certainly be outbid by United.
Fergie is believed to like the physical attributes the striker could add to his side after he lost Cristiano Ronaldo and Carlos Tevez last summer.
Hulk made his Brazil debut against England in November and now has two caps. He is likely to be named in his country's World Cup squad.
He got his nickname because of his stocky appearance and his likeness to actor Lou Ferrigno, who played the comic book hero.
"Hulk would be a good investment for United," said a Porto source. "He has the quality to be a superstar in England and his club will have no choice but to take a big offer for him very seriously."
Not a bad choice, but he's not even prolific at Porto and for 20million?! Are u kidding me? 35 competitive matches played and only 10 goals in return. Unless Fergie is looking for a supprt striker otherwise it's wise to look elsewhere.
Manchester United may have to wait until the summer if they are going to sign Hulk, according to the Porto striker's agent.
The Red Devils are reportedly lining up a £20million raid for the Porto forward, who boasts an impressive scoring record with the Portuguese champions and has made a number of eye-catching performances in the Champions League this season.
The 23-year-old is hoping to secure a place in the Brazil squad for this summer's World Cup in South Africa, and his agent Teodoro Fonseca is wary that a January move could disrupt his progress.
"Inclusion in Brazil's World Cup squad is the most important thing," he said in The Sun.
"If he gets in, it could act as the gateway to England for him and that is what we want."
Hulk, who spent three years playing in Japan before moving to Porto, made his debut for Brazil in November last year when he came on as a substitute in the 1-0 win over England.
United manager Sir Alex Ferguson is rumoured to be interested in a striker to relieve Wayne Rooney of the goalscoring burden at Old Trafford.