Fletcher, last season's leading scorer with 12 goals, has missed the last three league games with a calf problem. But the former Scottish international returned to full-time training on Friday.
McCarthy said: "Fletch trained with the squad for the first time and he did okay. If he is right, he will be involved on Sunday."
He added: "We are aware of the contribution he has made. But we will wait and see how he is before making any decision.
"The last thing we want to do is rush him back for this game and then repent at leisure during the two weeks of the international break."
McCarthy is confident playmaker Jamie O'Hara (knee) and midfielder Stephen Hunt (back) will also be fit against the Latics.
The duo trained on Friday for the first time since last weekend's 3-1 defeat at Manchester City.
McCarthy said: "They should both be fine."
Source: PA
Source: PA