in that fight, itachi didn't have his usual weakness of stamina, +two perfect jinchuuriki to save them from CT and provide distractions. Nagato not only had his usual disability, but he was further hampered by edo tensei.
in a fair fight, Nagato is superior to Itachi by a significant amount. Doesn't mean that Itachi doesn't have a chance at winning though, his abilities measure up well against a lot of opponents. But Nagato has like several times his stamina and options, that can't be ignored. Gedo Mazo alone would cause Itachi all sorts of trouble, and the summons. Itachi can't defeat them all without susanoo or amaterasu.