Ireland manager on whether he'll drop Glenn Whelan after Hearts exit
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The experienced campaigner signed a one-year deal with the Jam Tarts last summer but left in the January window after new manager Daniel Stendel made it clear he was not in his first-team plans.
The 36-year-old later accused the German of throwing him "under the bus".
Whelan found a new club before the end of the window, signing for Joey Barton's Fleetwood Town.
It was suggested when Whelan signed for Hearts he was motivated by a desire to keep his place in the Ireland squad as he approaches 100 caps.
Asked if playing at a level below the Scottish Premiership would cause McCarthy to have doubts about the player, the national team coach was emphatic in his response.
He said: "I’m not teaching Glenn Whelan any new tricks. I’m not showing him anything new.
“He’ll be running around just as hard in League One as he was anywhere else, so as long as he’s fit, and playing [he’ll be involved].
"I understand the questions when they're doing well in the Championship, and he [Milwall's Jayson Molumby] played well yesterday, been doing really well, they're really pleased with him.
"Glenn, for me, who has been the star man in probably three or four of our games, he has the edge on those, without any doubt.”