The 37-year-old veteran picked up a calf injury during Birmingham's 1-0 friendly defeat to Mallorca on Saturday after he had come on as a 74th-minute substitute for Cameron Jerome.
He hobbled out of the action at St Andrew's and McLeish believes the early signs are not good for the striker.
McLeish told Sky Sports News: "He said it felt like a bullet in his leg, his thigh muscle exploded.
"So his early signs are not good, it looks as if he could be out for a while with that one.
"Hopefully the diagnosis over the next 24 hours proves different but he'll be getting scanned tomorrow, well in fact Monday it will probably have to be now.
"It doesn't look good for the wee man and more importantly for the club as a whole."