He's not considered a threat on that surface by almost all but I'm not buying it.
the strange and trully great thing about Delpotro is that he is not the typical argentinian. clay is not his fav. surface, he prefers hardcourts, he nevers plays the local event in Baires and he has even asked to play Davis Cup in Argentina on hardcourts.
can he dominate on clay? probably, the asterisk is his movement.
he will probably take more seriously IW and Miami than the european clay swing (matchplay/practise then) and of course I expect him to be ready and on fire for the US Open series.
He took Fed to 5 sets and had a good chance of winning that match if he didn't tire out.
With that win he may have had a RG title in his pocket.
This is all before he really started improving into the hard court season, he never reached his peak as there were areas he was improving on, endurance being the one that allowed him to take the USO title.
He matches up well with Nadal and may have the game to beat him there.
His movement is good enough on clay, he has plenty of power to take control of points and a large wingspan to get to a lot of balls.
He is lacking in a lot of areas now and still is causing problems on the tour with just being back for a short time, he can get a lot better goes without saying.
These are just a few of the reasons he can do well on clay as well as hards.
On grass I don't know how his game will progress.