Serbian Djokovic has struggled to find his best form during a difficult few months having undergone elbow surgery.
The former world number one is seeded only 20 at Roland Garros, but he showed glimpses of his old self at last week's Italian Open before bowing out to Nadal in the semi-finals.
Nadal, who has lost just twice in 81 matches on the red clay of Paris, is once again the heavy favourite to triumph.
But he said: "Novak is one of the best players of our history of this sport. My answer is he's not coming back, he's there always.
"Sometimes when you come back from injuries it's difficult to play that well from the beginning, but he played a great event in Rome, and is no doubt one of the candidates to win here."
Nadal, himself only recently back from a hip injury, went on to win in Rome and begins his French Open campaign against Ukraine's Alexandr Dolgopolov.
"I feel good," added the Spaniard, who is back at the top of the world rankings.
"Of course after a very tough beginning of the season, with two injuries in the same place, it's true I came back and played well.
"I have played a lot of matches and had good success since I came back.
"But every tournament is different, and here we are to try our best, to have good days of practice before the tournament start, and try to be very competitive for the beginning."
Meanwhile, former Wimbledon quarter-finalist Bernard Tomic of Australia, now ranked a lowly 208, reached the main draw after winning his final qualifier.
Only four months ago Tomic was appearing as a contestant on the Australian version of 'I'm a Celebrity...Get Me Out of Here'.
But the 25-year-old will be back on more familiar territory next week following a 7-6 (7/5) 7-5 win over Portugal's Goncalo Oliveira.
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