Alekhine was in pretty bad shape by then. He hadn't played in any tournaments but local Portugese or Spanish events since 1943. He was in his 50s, and not in the best of health.
I would expect Alekhine to win a short match, but Botvinnik to win any protracted contest thanks to greater stamina and staying power.
Who would have won, if Alekhine had lived to defend his title?