Celtic this week rejected a second offer for the former Scunthorpe player and manager Neil Lennon revealed the latest figure was "nowhere near" £6million.
"He is only 24, he is scoring goals in the Champions League and the Scottish Premier League. He can do no more than he is doing," McGarvey said. "He could be getting a call-up into the England squad soon because he is scoring goals in the Champions League, which is the top league in world football."
He added: "For English clubs to offer £5million is an absolute joke."
McGarvey, who left St Mirren for Liverpool before joining Celtic, added: "I wouldn't go to Norwich. But, if you are getting double the wages you are getting at Celtic, it would be very difficult to say no.
"What person in the world would say no to double their wages if they went to England?
"It's very difficult for Celtic to compete with the English Premier League because they have so much money, but Celtic have a bit of Champions League money and have held on to their players well in the last couple of years and have made more than £20million from the Champions League because of that.
"I think Celtic will try to hold on to him.
"If he scores against Juventus and helps Celtic into the last eight, that would make Celtic another five or six million. You have to look at the agent, he will be egging him on to go because that makes him money. He will be getting pressured from both sides.
"If Manchester United came in and offered £15million then I would think about that but if he stays at Celtic he will be in the Champions League every year and pit his wits against the best players in the world."