Howard's form has been indifferent so far this season along with almost every other player besides maybe Mirallas, Baines and McCarthy. But for the previous 18 months he's been very consistent and one of the top keepers in the league, he's not long signed a 3 year contract extension which indicates Martinez isn't looking to replace him any time soon, unless of course an exceptional offer is made.
It's very unlikely Martinez is going to change a 3/4 year plan of squad managment and overhaul because Howard has had a few below par games so far this season. To upgrade on Howard at the moment we'd be looking to sacrifice the next phase of signings that Martinez thinks would improve the squad and move us forward, it's clear given the contract situations and their respective ages and form that the next major signings will be a new central defender and an attacking/creative midfield player. These positions are by far more pressing and in need of strengthening before our number 1 goalkeeping position.
The best we can hope for is a more competent and reliable young understudy to come in who can challenge and maybe replace Howard in the long run, or hope Robles makes a huge leap and kicks on. In short don't moan in twelve months time when either Distin or Osman are pulling on a shirt again for the second game in 3 days and our new £12m+ keeper gets beat at the near post.