I don't think it'll hurt his momentum. He'll still be booked to come close to destroying Triple H in the cell or whatever and will thus look strong even ending up on the losing end of the match. And just as a reminder, Brock was gonna lose anyway. If he had ended up in a match with Undertaker like everyone wanted, he would have lost. If he ended up in a match with Ryback (some said he'd be a great guy to help put Ryback over), he would have lost. Losing to Triple H might annoy many because of HHH just having to get his win back, but it won't hurt his future drawing power at all. People said the same about the surprising Cena loss, but then Summerslam went on to do an impressive buy rate. And Triple H's probable win at Mania doesn't change the fact that he tapped cleanly at Summerslam.
He's supposed to feud with CM Punk later in the year. That could be a tricky feud to book since I don't think either guy should lose. Brock should win his next match/feud (perhaps his next two, don't want to make it look like he's just here to do jobs), but so should Punk. Him taking time off after Wrestlemania (assumption) after a string of losses only to come back to another loss doesn't sound too good either. Maybe they can book around it by having their first meeting a normal match that escalates into an all-out brawl that ends in a no contest. Their next match is a last man standing match, that ends when they're both down and then one of them just barely manages to get up before the last count. This is a better way to job either guy since it comes down to a close call and neither guy eats a pinfall or taps out.
Let's not forget that Brock is losing to The Rock at Wrestlemania 30. I personally think between now after WM29 and then, he shouldn't take a pinfall of submission loss.