The Brazil Under-20 international joined the Latics on a three-year contract from Chelsea minutes before the transfer deadline closed on Tuesday.
His permanent move was something of a surprise but Martinez is delighted he is now able to work with the player long-term rather than just for one season.
"It is exciting because in any loan situation you do not have the bigger picture and the long-term ambition," said the Spaniard.
"It is not just a matter of getting him up to speed and then losing at the end of a 12-month period.
"It is important that from all the young players we are developing here we get rewards.
"To have Franco here as a permanent player is a great signing for us as we can really focus on his development and fulfilling his potential so Wigan can get the rewards.
It was a clear statement that we wanted the player for the long run to make sure he can have the best years of his career at Wigan."
Di Santo arrived in England from Chilean side Audax Italiano in 2008 for £3million but made just 16 appearances for Chelsea and never started a game.
He went on loan to Blackburn last season, playing 22 times in the Premier League but scoring just once.
But Martinez said rather than being put off by that apparent failure to nail down a first-team place, he was encouraged by it.
"His spell at Blackburn was probably the reason why we were so certain about him," added the Wigan manager.
"He needed that period as a young player. When you go into the Premier League the first 20 games are huge in terms of experience and knowing what to expect and preparing yourself for the next chapter.
"It is not about how he did in that chapter it is about how he matured and found a way to help his team.
"I just feel now he is ready to go into the Premier League and be himself and you only do that from experiences.
"The hardest thing for a striker of the type of Franco is to have Premier League experience.
"For me the time he had at Blackburn was the final push to make the decision to sign him permanently.
"He now has 20-odd games in the Premier League and comes here as someone desperate to earn his place and be himself.
"If you speak with him he is desperate to enjoy his football and be settled."
Martinez suggested the pressure of trying to prove himself to Chelsea had weighed heavy on the 21-year-old but at the DW Stadium he would be able to play with more freedom.
"He has always had that dream of earning the chance to play for Chelsea and now he can focus on playing for Wigan and being himself," he added.
"I have seen him play many games for Audax in Chile and all I want is for him to recover that level and if he does that we will enjoy a lot of good football from him."