The club's joint-record signing, who joined for ?6.5million from Burnley last summer, suffered the injury in the 2-0 home defeat to Tottenham and is facing a race against time to be fit to face Neil Warnock's top-flight newcomers.
McCarthy told the club website: "Fletcher has hurt his groin and is a doubt for Saturday. It looked really bad on Sunday but it had settled down yesterday (Monday). We are hopeful, that's how I would rate him."
McCarthy also revealed that Stephen Hunt and Kevin Foley will not train until later in the week, although he expects them to be available to face the Hoops.
"Kevin Foley has an ankle problem and he won't train until Thursday - he's had to have an injection in his ankle." added the Wolves boss.
"Stephen Hunt has had to have one as well and he won't train until Thursday."
Source: PA
Source: PA