If they think he can be competitive in TDF and Vuelta he should do the TDF. If they think riding TDF will make him a bit too tire for the Vuelta, he should ride the Giro. I would focus on getting that 5th Vuelta win, it's a realistic goal.You can't wait until the end of August for a rider like Roglič to ride his first GT of the season. He'll definitely do La Vuelta, the only question now is Giro or Tour.
