Watson came on as a substitute after recovering from a broken leg, and Martinez said: "The whole journey of Wigan Athletic in this FA Cup has been about little stories like that.
"Everybody told us in the medical department Ben was going to be out for the rest of the season. But he wanted to get back. He worked extra hours and pushes himself, then comes on at Wembley and scores the winner.
"I've seen movies with worse scripts than that. It was fantastic to see him scoring a goal.
"The proudest thing for Wigan Athletic was that we didn't perform in a damage-limitation way and then get a break. To produce that level of performance and in my eyes be the better side over the course of the game was very, very pleasing."
Defeat meant City ended the season without a trophy, and the match was played against the backdrop of intense speculation about the future of manager Roberto Mancini, who was duly sacked two days later.
Elation quickly turned to despondency for Wigan, too, as they were relegated from the Premier League the following week, with Martinez subsequently leaving to join Everton.
A statue of Whelan holding the FA Cup was unveiled outside the DW Stadium in 2016. Two years later the family sold the club, with administration following last summer, and the Latics narrowly avoided relegation to League Two this season following a takeover by the Phoenix 2021 consortium.