The Scotland forward has been utilised in a wide position rather than his preferred central role in the last three defeats to Tottenham, Aston Villa and Wigan.
McCarthy believes the team has benefitted with Fletcher playing wide and he insists the current formation has plenty of flexibility.
"He scored 12 goals at Burnley last season playing wide most of the time because they played 4-3-3," he said.
"And for a spell at Wigan he went up front on his own and (Kevin) Doyle went wide right.
"Fletcher was brilliant against Villa in the second half.
"He kept the ball, came inside and passed it - it was a really good performance.
"And he scored his goal at Tottenham playing wide left.
"It's not wide left or right, it's 4-3-3. The front three have to inter-change."