You are using Messi as an example, but it isn't like it wasn't one of our worst mistakes that we made this habit of allowing him to make such decisions. At 25 Messi has adjusted his game into a guy walking who only run at spurts, at the time it was acceptable, in football now it is a liability. At least he had 3 CL by then, Lamine might not get there.
And Messi got injured enough early on to use it as his own rest. Add that to the fact that Messi was entirely different beast, not by how good he was early on, but with his style. Messi style was by avoiding contact and touches, plenty of players have spoken about how fouling him was equally difficult to disposing him, while Lamine style is just like Neymar and Ronaldinho, it invites tackles and fouls. In a league that literally has no intention to protect him by fair refereeing.
Roony was a viable backup this season, Rashford was viable backup to Raphinha and could have allowed the Brazilian to move to RW sometimes, it barely happened. Rashford himself is capable of playing RW, and we never do it.
And atm, I've no reason to trust that the medical and training staff doing the right things with him, we have a long history of young players suffering too many injuries to not be legitimately worried. They claimed last year that Pedri needed extra load, and are repeating it this season while he is clearly suffering.
This is simply a bad rotation, while simultaneously installing bad habit in a young player, on the expense of his teammate.
Sure, Roony will be loaned or sold, but I won't get my hopes up about a La Masia backup, that sound like wishful thinking. Flick has given the opportunity to essentially only 3 players from the reserve team, Martin, Bernal & Casado, two of those were older prospects, the other was a physical demon for his position, all in his 1st year only.
Nomoko will be a young guy coming out of ACL midseason to join our likely 4th division B side, I won't get my hopes high. Even the guy Flick was obsessed with has left because he wanted more minutes.
imagine telling Messi under Pep in his 3rd season to sit?
Why imagine, though?
On his 3rd year, Messi started only 5 out of 9 CdR games, sitting completely in 2 of them. He played less than 65% of CdR minutes.
This season, Lamine played 100% of the CdR minutes.
Same pattern with using Joan in all games he was available, zero rest or giving Tek the CdR.
I don't think you realise how allergic Flick himself to rotations, and you are just assuming it is all Lamine. Of coarse, he won't say no to play games, but Flick is more than enabler in such case. It is just his mentality of going all in all the time, expecting unhuman things from his players and failing to realize the difference in load between Bundesliga and La Liga.