Valbuena has questioned Mbappe's commitment to France, with the striker out of Les Bleus' squad this week.
Now with Athens Kallithea, Valbuena told RMC Sport: "Being captain is not just a piece of cloth around your arm, it also means having responsibilities on and off the pitch.
"And I think it would have been important for him to be there for those games, even though he knew he was tired. He could have been there for the group, that's also the captain. He's also part of the French national team, he could have been there and his decision is quite strange.
"What surprises the French national team fans is the fact that he is not in the selection and that he would play this weekend against Villarreal. Mbappé's priority, I think, is not the French national team but Real Madrid. He arrives at a new club, he knows he has everything to prove and that it is not easy to do it there. I think he is giving everything he can to do it... We saw his last outings with France, he was not a great Mbappé, even in the way he played, we had the impression that he was not there"."