If he really thought Michael was going to make Carlo his right hand man then he wasn't very smart. I have always felt that Michael liked and trusted Clemenza more then Tessio. Even before they know where the meeting is going to be held its Clemenza that Michael trusts to plant the gun. It's Clemenza that instructs Michael on how to carry out the hit. When Clemenza is cooking for the men while Michael is talking to Kay you even see Clemenza joking with him about telling Kay he loves her. You never see any of this one-on-one interaction between Michael and Tessio. I also think Michael felt Clemenza was more loyal to his father and the family. When Vito asks Clemenza and Tessio if they trust him and Michael's plans its Clemenza that immediately replies " yes Godfather." I agree with some of the others that the"Tessio was always smarter" remark was said sarcastically. In my opinion I think Micheal expected the traitor to be Tessio the whole time.