No he isnt. He has played as a CB during his last season at Bilbao and he was selected in the team of the season in PD that year, but Bielsa is prob a clown too.
He did a great job in his 1st year partnering BS31 and forming a midfield made of steel, but Bayern have developed since and such a defensive midfield wasnt needed anymore. Besides the DLP a more offensive player was needed with better technique and passing abilities to strengthen the possession game and players like Kroos, Thiago and Vidal are/were much better suited to such a role.
BTW Martinez is a fantastic CB and he has been preferred to Benatia easily, as he is just a very complete player..his calmness and opening play is much better. During his last 5 seasons, he has played 4 as a CB.