Funny isn't it - Guardiola never rested him, never injured in four seasons (bar Ujfalusi's disgusting tackle). He's been rested a couple times this season and he's suffered two injuries. May that be a lesson to the muppets who consistently harp on about how he needs to rest, it's became tedious in the fact EVERY single week it's being said on here.
Well you can't draw conclusions like that. First thing is he got older. The second thing is that playing a lot puts strain on your body and sooner or later you will injure yourself, for Messi it became later. Now true, we don't know if Tito changed his diet and training but there are still many factors that decide wheter or not a injury might appear.
Things like bad luck is probably the culprit here, no matter how good genes or how fast you heal if you have bad luck you will have a bad time. Guardiola did rest him when he was able to but mostly they were rarely this dominant to be able to rest Messi without risking a title, they always had important point games or just important games therefore resting wasn't an option. Also one of the reasons Iniesta injured himself and was gone for a good while and was experiencing bad form.
Usually rest seems to motivate and make Messi a monster i.e. after holiday breaks he comes back strong or in the beginning of the season. I don't remember after Ujfalusi´s tackle how he played when he came back, but this time I think he has to have at least one game he plays to both test how it feels when it doesn't matter much and also gain confidence.