I forget where I saw the comment but I remember reading something interesting about how Messi's game is becoming more suited for beating the lesser teams than dominating big matches. In the big matches, it's impossible for a team (other than Pep's barca) to completely dominate the midfield enough to supply the forwards with constant perfect service into feet with space. As a result, pace and off the ball movement (two things Messi is starting to lose) become a lot more important.
Against 80% of teams in La Liga, this doesn't really matter though because our midfield is dominant and we have so much possession around the box that Messi will still get enough space and enough of the ball to do his stuff. But in the big matches, I don't think Messi playing this static #10 type role in a 442 works very well. We should either play 433 with him upfront through the middle or as that right inside forward type player where he did so well in 2015.