Let me preface this by saying, this is 100% the right move. There is no reason to try to salvage a season with Charlie Whitehurst or a banged up Jake Locker. Zach Mettenberger is the prototypical Whiz QB, and his love for the tight end mixed with his downfield throwing ability should make this team a lot more efficient on passing downs.
Having said that, there is one big issue that Titans fans need to brace themselves for on Sunday: ball security. Mettenberger may throw an interception or two, but his potential back breaking turnovers will be if someone strip sacks him. Obviously this goes for all QBs, but Mett is prone to do this more than most given what fans saw in preseason.
With Watt, Reed, Mercilus, and Clowney on the field, there is a great chance that the Titans face some disruption in the pocket. If Mettenberger can go all game (playing like he did in preseason) without getting hurt and without fumbling, I think that is a huge step forward going into the bye week.
This team needs a quarterback that can stretch the field, but also take advantage of Delanie Walker's ability, and (if you replace Walker with Taylor Thompson) that is exactly what he did in the preseason. This is a game where Mett won't have to worry too much about the defensive backs in Houston, and should be making quick reads. If he can do that, Titans fans should be happy regardless of the outcome.