He can't play on clay. His movement is poor and his baseline game is not strong enough.
Beating G. Granollers, Di Mauro and Arnaboldi really doesn't change that assessment.
And beating Darcis on clay isn't impressive?
Look, I'm not saying he's a world beater on clay, but he's more than capable of holding his own. FWIW, he was robbed today in the first set tie-break.