The fly-half had previously been with Ireland at the Rugby World Cup and began the clash in Italy by converting Jack McGrath's early try.
Sexton was then sin-binned after ten minutes and when he returned, Fergus McFadden had taken over from the tee - but Cullen insists it's nothing to get excited about.
"It was just when he came off, the fact that he cooled down," Cullen told The 42. "Ferg has been kicking - again, it's just erring on the side of caution.
"It's good that he came back on and played until we had scored four tries at 27-3. Then I wanted to get another look at Cathal Marsh and Luke McGrath at half-back.
"I thought they looked really sharp when they came on, so good to see them really pushing for their place in the team."
There were also lingering doubts over Luke Fitzgerald, who was withdrawn shortly before the half-hour mark at the Stadio Monigo.
"It was more of a precaution because of the history Luke has had with his neck," added Cullen
"We had Noel Reid on the bench as well - Noel has played all the games at 12. I know, 30 minutes in, it's not ideal because it limits your options later. But with Noel there, it's an easy solve.
"Luke will get assessed and see when he will get back in to the action."
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