So Messi has a (very good) performance against Sevilla just two weeks after getting embarrassed by Mbappe in the Camp Nou and doing nothing against Sevilla away (the same Sevilla Dortmund, 5th in the Bundesliga), defeated 2-3 away) and he's suddenly the best player in the world right now? Christ. Mbappe ended that conversation when he annihilated Messi's barca in our own stadium and made Messi look like what he is: a 34 year old player well-past his best.
I don't buy the 'weaker team' argument because your claim is that Messi is, right now, the world's best. If the world's best can't impose himself in a match, then what's the point of being the best? If you are going claim Messi is 'the best' then he better perform like the best. Part of being 'the best' involves being decisive in important matches. Messi's failure to do exactly that mean's were effectively out of the CL and almost out of the Copa.
If your claim is correct, then: 1) Messi has to eliminate Sevilla mid-week. We beat them yesterday; it is entirely possible. 2) Messi has to have a good, solid performance against PSG. I don't mean he has to score a hattrick in Paris and take Barca to the next round, but he has to impose himself by creating chances, perhaps scoring a goal, and generally making PSG sweat. 3) Messi has to perform--either assisting or scoring--against Atleti and Real in the weeks that follow.
I could care less if Messi scores against mid-table and relegation teams. Being the best means making a difference when it matters and the stakes are high.