And in what are you basing this assumption?
I would choose Delpo any day for a clay match. Except if it was RG final.
On Del Potro being the better player and having a much more suitable game to upset Nole and the belief - odds are even if Ferrer got close to pulling the upset, he'd end up choking it. No reason he'd get close anyway as Nole isn't troubled by him in the slighest ever since he became the dominant player back in 2011.
Remember their 2011 match at RG months mere months after JMDP returned to the tour, Delpo was more than matching Nole before the match was interrupted due to lack of natural light; I'm pretty sure Novak will have his hands full if he faces JMDP at RG, especially as he can't defend against his power shots as effectively as on a hardcourt.
Last time they played on clay, Djokovic barely escaped in three and then beat Nadal in two. Ferrer is a hard match up for Djokovic on clay. On Hardcourt, he is a cakewalk though.
Mark Lenders, I know you are a big Del Potro fan and that you dislike Ferrer, but Ferrer is, without any debate,better than Del Potro on clay. By titles won, by H2H, by SF and F reached and overall results.
After 'barely escaping' Ferrer, Djokovic had an even closer call against Bellucci, I suppose Thomaz is also a bigger threat for Djokovic than Del Potro, eh?