Martyn, who made 23 appearances for England between 1992 and 2002, thinks the 21-year-old goalkeeper has rediscovered his confidence following a tough start to his career at Old Trafford.
He said it had naturally taken him "a little bit of time to adapt" but added: "Working at it as he obviously is, it's bearing fruits now. His form has coincided with Manchester United's form at the end of the season and it's on that basis, a good defence and a good goalkeeper, that league titles are won."
De Gea has been under the spotlight after he replaced the retiring Edwin van der Sar, especially following some shaky performances in the early part of the season.
However, the Spaniard was unflappable in Monday night's 2-0 win over Blackburn at Ewood Park, tipping over Marcus Olsson's strike at the end of the first half before saving Grant Hanley's header from the following corner.
The performance was in stark contrast to his display against the same opposition at Old Trafford on New Year's Eve where he was subsequently dropped, and Martyn believes United's current success is closely linked with De Gea's improvement.
"He's a young man coming through a foreign country, playing in a different league and it's obviously going to take him a little bit of time to adapt," Martyn said.
He added: "Possibly his mistakes were coinciding with the period of time that Manchester United didn't look as if they were realistic title challengers, but what has happened is his good form has coincided with theirs.
"It's probably not a coincidence that the two things have gone together and they've kicked on and really are odds on favourites to win. I think it's definitely his form that has enabled the rest of the team to gather their form."
And Martyn, who ended his professional playing career playing for Everton in 2006, praised De Gea's attitude in overcoming his shaky start despite the glare of public and media attention as "impressive for somebody so young".