Messi was a great player for Barcelona, prolly the best to ever come out of camp nou, but I'd not say that he singlehandedly carried Barcelona to glory, I know he was their most important and influential player, But Barcelona is a great club and they had a lot of quality players, Messi made them absolutely complete, but he wasn't everything, he also needed other players to function as well, what I mean is, Barca and Messi had a seamless relationship. Just take a look at how things are panning out in PSG for Messi, he isn't at his best cause he does not seem to have the sort of connection he had at Barcelona. Having said that, I still don't think PSG are somewhat ready to win the CL, they have the individual talents, but as a team, they are not the best and it could still take them a while before they are CL champions.
Messi wasn't singlehandedly carrying but he had the most impact on the pitch for Barcelona. We also cannot deny the impact players like Xavi, Iniesta or Neymar had but Messi had the most impact. Before Neymar and Suarez joined, he had the duty of scoring goals and he did that very well. Even scoring 91 goals in a calendar year.
(I dunno how he did that) After Neymar and Suarez joined, he moved to the right flank and assisted them with tons of goals also while he scored lots by himself.
@Coyster, yes agreed, not singlehandedly but he was the most important player in my eyes. I'm not saying that Iniesta or Xavi didn't do anything for Barcelona but if we look at the larger timeline, with Xavi/Iniesta and after the Xavi/Iniesta era, Messi is the most important one. Even before he left, Barcelona were in a good state. Their attacks were good,, the build-up was good. But after he left, it seemed like Barcelona forgot to build up to the attack.