No one would question Vito's choice, but the novel points out that many (in and out of the family) thought Michael put in "a bravura performance against the Turk and McCluskey," but he lacked "force." Tessio obviously carried that thought to the extreme when he betrayed Michael.
All true TB. However, as the novel states, "Tessio had a better opinion of Michael. He sensed something else in the young man: a force cleverly kept hidden, a man jealously guarding his true strength from public gaze ..."