The 22-year-old has started to move up the rankings this year and has gone into Wimbledon 31st in the standings.
Novak Djokovic, Roger Federer, Andy Murray and Rafael Nadal have dominated the Grand Slams for a number of years now, but McNamara, who coached the Bulgarian for 12 months up until 2011, has told Sports Mole that Dimitrov has the potential to force his way into that group.
"He's the next one. I can't see too many others [doing it]. They are just not knocking on the door. He's made a move this year and won a few matches. Beating Djokovic on clay in Barcelona is a great effort," said the former Australian professional.
"He can play on all courts, although grass is his best opportunity. If he is confident, he can beat anyone on his day. He's certainly the next man that will break into the top five or six."
You can read our full Wimbledon preview with McNamara here.
Stella Artois was the official beer supplier of the BNP Paribas Tennis Classic at The Hurlingham Club, an ideal setting for legends of tennis to battle it out at the prestigious event.