Guillermo Coria was a very specific player. He was a great clay court player and not all that bad on other surfaces but there are mainly two issues with him which I would like to point out :
1 - Coria is a fairly small guy standing at 175cm. In my honest opinion this is a huge disadvantage in tennis especially in a matchup vs Rafa. The speed and movement which these smaller guys have doesn't give that big enough advantage compared to disadvantage of lacking the power and strength. You can not blow Rafa off court on clay with 175cm and you can not win being defensive, that much we all know.
I will add another proof to this by putting Nikolay Davydenko in the argument. He was one of the guys who was smaller, good on his feet and with excellent clean hitting and execution. He was awkward matchup for Rafa which their record shows ( 6-5 to Davy ). Everything being said, he only came close to Rafa on clay once and afterwords he said ( as did Ferrer any times ) 'I have no stamina to endure a match vs Rafael on clay'.
And another fact - to this day the the only player ever to beat Rafa on RG is actually a powerhouse with a huge serve and groundies.
2 - Coria featured in 20 finals losing 11 of those. Mentally, to beat Rafa you have to be a giant. To beat Rafa on clay you have to be in godlike mode. To beat Rafa in RG in his prime years ( when Coria would be facing him ) you'd have to be ... you get the point. So did Coria's game suddenly fell apart by accident? Well no, he was just not that strong to overcome defeat in RG final, injuries and in my opinion the rise of Rafa did no good to him either.
In my mind Gullermo Coria could have beaten Rafael Nadal in RG but that could have happened only in 2005, only if he had previously won it in 2004 and only if Rafa had an extremely bad day. Nothing such happened and Coria bumped into Davydenko in 2005 RG, who as I pointed out was not a guy you like to see across the net on clay.