I think he can't keep his level over the season because he doesn't shine in the core attributes required for this position. To be a great fullback consistently you have to be fast, strong, aggressive, and very good in 1 vs 1. Roberto's strenghts are all secondary in this position, the sort of stuff that the great RBs have ON TOP of the core qualities. Roberto in the end is not a RB and his skill set is only a partial match with that position, and the part that doesn't match is too important to leave aside.
It's better and easier for us to go with Semedo and improve his secondary attributes, knowing that he is fast, strong and aggresive enough to keep us solid in defense. Than to use Roberto's bonus skills, having to cope with his issues which are serious deficiencies for a RB.