Messi does this quite often, I think it is because he knows Dembele would further the attack and either go for a goal or pass it to someone who is free. However, Messi would rather pass to Pedri or Jordi Alba as they always give him back the ball
I can still give Koeman a chance (he subs players when they underperform). He would probably have full control of the team when Messi leaves then we would see what he really has to offer (when he has bought the players he want)