It will be tough when Rafa retires
I don't know if will be somebody with the same passion, intensity and mental strenght in his game. Rafa is unique for a lot of reasons, it will be tough to assume that he will be a retired player but now it is a little bit early to think about it, and specially when he has returned to play after a long rest (to avoid more knee injuries) that has being a good decision (and this maybe makes Nadal career longer
). We have some years with him rocking on clay
Roland Garros should be not a very difficult tournament to win because in a best of five there is no player in the world that can beat Rafa except Djokovic (in fact in a best of three Rafa can be closer to be defeated by another player but anyway he always wins with his gladiatorial spirit of fight and his hate to loose). Nole is the only 'big' threat that Rafa has. With the level of tennis of the last year Rafa was favourite against Novak in RG (and in fact without rain the final of RG 2012 would have been Rafa in three sets in the same dinamic of the first two sets). But all we know that this year Rafa is returning on courts and playing a better tennis day by day but his court coverage and mobility are worse than before. Now Djokovic finds his strategy to beat Rafa more succesful than before: atacking him one or two times in the backhand zone (trying to do it in a way that ensures that Rafa will answer his shot with the backhand and not with the forehand) and then shot a cross court backhand or a inside out forehand to the other part of the court. Rafa has to run a lot and he is not as good as before in his shots on the run. Moreover, he has pain in the left knee and he has changed a little bit the footwork in order to put more weight in the right knee so it will take time to see Rafa with better footwork.
In short, there are a lot of factors that say us that Djokovic can win Rafa, but we are in Roland Garros and the pride and spirit of fight of Rafa will be always the most powerful weapon that anyone can have in a surface like clay. Nadal is the living legend in the tournament and sometimes I believe more in history, tradition and this kind of things more than in real facts. Montecarlo's defeat was tough but Roland Garros is a Grand Slam. I am not saying that Rafa will loose if he meets Djokovic, but I think it will be 60% to Djokovic if Rafa doesn't improve his game and Djokovic shows a better version than in the last two tournaments (mentally speaking because he played well but not focused in win points and close matches).
Anyway, it doesn't matter if we think that Rafa can win against Djokovic or not. We will have to trust in him as we always do, and cheer him if he wins or if he loses, as Federer now he has proved that he is the better clay courter of tennis history and if he loses to Djokovic nothing won't change