The Scot hit the second of his side's three goals in a 3-1 victory over Martinez's former club as Everton booked their place in the quarter-finals of the competition.
And Martinez revealed Naismith had been left somewhat confused after a nasty collision with Swansea defender Jordi Amat forced him from the field in the closing stages.
"Steven has concussion and couldn't remember scoring," he said, before joking: "I've told him he scored from 40 yards."
The Spaniard handed a debut to Monaco loanee Lacina Traore and the Ivorian opened his Everton account in the fourth minute of the game.
"For Lacina to play 60 minutes and get a goal is fantastic news for us," continued Martinez. "It's a good sign for the future.
"We're developing into a side that can get into the final third very well. It's what you do when you're there that will win games.
"I thought we made very good decisions in the final third today, which is pleasing. It's an area where we need to improve."
Jonathan de Guzman scored Swansea's equaliser, while Leighton Baines rounded off the scoring for Everton from the penalty spot.
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