The Scotland hitman broke a metatarsal in Friday's 1-0 win against Perth Glory in Australia.
Iwelumo was playing his first game since injuring his medial knee ligaments against Birmingham on April 6, and McCarthy moaned: "He has broken his metatarsal. I don't know whether it will be six, eight or 12 weeks, some heal quicker than others.
"I'm hugely disappointed, for Chris more than anything. Chris has played all his career in the lower leagues and worked his socks off to get this opportunity of the Premier League and then he breaks his toe."
McCarthy, meanwhile, insistsed former Leeds midfielder Jacob Burns, whose clash with Iwelumo caused the injury, was not to blame.
"It wasn't a physical game - it was an accidental clash," the boss told the Express & Star. "The game was fine - there was no badness in it at all."
McCarthy gave debuts to new signings Kevin Doyle, Nenad Milijas, Marcus Hahnemann, Andrew Surman, Greg Halford and Ronald Zubar, with Surman catching the eye.
But he was far from happy with the performance, adding: "We were hopeless. I hate pre-season."