The 23-year-old has forced his way onto the fringe of Fabio Capello's England squad after numerous stand-out performances for Wanderers since his £5million move from Aston Villa in January 2008.
Capello was in the crowd to watch Bolton's 2-1 victory at Birmingham on Saturday and Cahill has already been linked with Tottenham, AC Milan and Juventus.
The centre-back has twice been called into the national squad to cover for injuries but could earn a place on merit when the Italian coach makes his selection for the forthcoming World Cup qualifiers against Ukraine and Belarus.
Megson moved quickly this summer in getting Cahill to sign a new three-year contract, removing a reported £8million release clause, and has no intention of letting his defensive linchpin leave.
"This club got itself into a nightmare scenario over Gary,'' said the Wanderers boss. "There was a set fee that he could have gone for.
"We did brilliantly in the summer to get rid of that and we're in a position now where we don't have to sell him. Football-wise it would be suicide to sell him in January.''
Megson knows the damage losing key players in the winter transfer window can do after selling Nicolas Anelka to Chelsea for £15million - although he used some of that cash to buy Cahill three weeks later.
"Selling real assets in January is a nightmare,'' Megson told the Bolton News. "We've been there before with Nicolas Anelka, didn't win in 10 games and escaped relegation only by the skin of our teeth in the last five games.''