We would probably need two centre backs to win the Champions League, but domestically? I don't agree, Smalling is one of the best centre backs in the league and Rojo and Jones are good enough cover.
I think the last point is exaggerated a bit. Forgive me if my memory is skewed, but did Smalling really miss large portions of the season like many are citing? He has spells out yes, but wasn't as injury prone as Evans, who must be shifted, and Jones who needs more competition. I have faith in Smalling as a first teamer, he's grown as a player and leader. One centre back will suffice IMO.