Dubliner Reid, 27, underwent a rigorous fitness regime in the summer to cut down his notoriously excessive weight.
Bruce maintains that is a credit to the former Tottenham and Charlton man.
"I do not think anyone has questioned his ability, but the one thing which has been levelled at him is his physical condition," said Bruce.
"He has now lost something like nine kilograms, 22 millimetres of body fat, which has taken him down to what you have seen today.
"He has always been blessed with ability, and now has got himself in a condition which you have to be - I think the penny has dropped that you have to be that way to play in the Premier League."
Bruce added: "Fair play to him, he is the one who has done it, I am not going to take any credit for that.
"Over the summer, he has worked hard and watched what he has put in his mouth.
"We all know what to do, but it is very difficult."