His credibility won't be lost at all. Triple H's cred wasn't either even though he lost his last two matches. Nothing will change that Triple H tapped out cleanly in their first encounter. Guys like Lesnar, Triple H, Cena, Rock and Undertaker are almost bulletproof when it comes to losses, they would have to job several times in a row before people start to think they're no longer that credible (and even then I'm not sure, but people will get burned out on them returning for big matches after awhile. The aura and novelty can only last so long.)
Lesnar will likely face The Rock at Wrestlemania, given how much hype and mainstream attention that match would get. I just pray it isn't for the WWE Title, what with the rumor today of Lesnar winning the title.
It's hard to predict who all those guys will face because it's a year away and who knows what level guys like Ryback, Ambrose, Reigns, Rollins and others will be at by then. Any of them could end up getting a match with one of the established elite.