McCartin's positional switch has been one of the most notable differences in the Swans' approach this season, Cox's first campaign in charge.
David King and Kane Cornes are among the AFL pundits to have questioned the merits of persisting with McCartin as a forward.
The former key defender has been used predominantly in attack during Sydney's season-opening losses to Hawthorn and Brisbane.
Cox resisted the temptation to send McCartin to the back line in the third quarter, when the Lions seized control of a contest they won by four points.
But the 25-year-old, who finished with one goal, seven marks and 12 touches at the SCG, was redeployed in a frantic final term.
"Tommy started really well," Cox said.
"When you're talking about Harris Andrews and the like, who match up on him, you've just got to provide a contest.
"He goes back as well, that's the luxury of having Tom in the team at both ends.
"It'll be horses for courses as we go throughout the year ... throughout a game, whether he's starting back and I'll swing him forward or vice versa like tonight.
"He knows defensive systems, he knows the craft."
Key defender Joel Hamling, who was previously on the books of Geelong, the Western Bulldogs and Fremantle, joined the Swans as a free agent at the end of the 2023 season.
Hamling made his club debut on Saturday, impressing Cox with the way he attacked multiple contests.
"We got him here for a reason - to play defence the way he did tonight," Cox said.
"He's been here for a period of time. He had the patience and the resilience to be ready when that opportunity came and he took that opportunity ... he played his role really well."
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