Barring injury or some kind of epic form meltdown on par with Chuck Knoblauch getting "the yips" and forgetting how to throw the ball to first base, Tim Howard will be your starting goalkeeper for Everton when the season opens in mid-August. There will likely be players brought in by Martinez that have a more obvious impact on Everton's season, but I'll contend that Joel Robles could very well be the most important.
The difference going into the 2013/14 season compared to the last few seasons is that Howard finally has a legitimate backup goalkeeper to push him for the starting job. New signing Joel Robles, who joined Everton from Atletico Madrid, is a 23-year old keeper with solid potential and the proven ability to hold his own in the Premier League, or at least the FA Cup.
He's not the next Iker Casillas by any stretch of the imagination, but Everton really don't need him to be. They need him to be out in training every single day forcing Tim Howard to be at his best. The more pressure Howard is under, the better he will likely play and that's a net positive for the Toffees.
Martinez knows this and he made the intent clear in an interview with the club's website.
"He [Robles] is someone who is going to give us great protection and fight with Tim Howard for the goalkeeping position and push him all the way."
Jan Mucha simply wasn't cutting it because he was clearly inferior to Howard when he got a chance to play and his attitude reeked of a guy who regretted signing with the team in the first place. This allowed Howard to stagnate to a certain extent, become comfortable in his job security.
While the US International keeper was still effective, there were plenty of glaring mistakes last season that caused some to start asking questions about Howard's status going forward. With Robles on board, Howard will now be pushed and either he'll respond or he won't.
Whatever happens, it will be a positive result for Everton because having Howard playing to his full potential or realizing that he's lost what it takes to be a top flight EPL keeper is valuable. Mucha wasn't doing that for the club and hopefully Robles will.