The American, who ruled the claycourt grand slam in 1991 and 1992, has watched the Spaniard's increasing vulnerability on the red dust with interest in the build-up to the year's second grand slam which begins in Paris on Sunday.
While Courier believes the best-of-five-set format means Nadal remains formidable, he makes world number one Novak Djokovic favourite followed by Japan's Kei Nishikori.
"For the first time in many years Nadal will not be the favourite going into Roland Garros," Courier, who will commentate on the tournament for British channel ITV, said.
"That role is now firmly occupied by Novak Djokovic. Nadal is less vulnerable in Paris, given the best of five-set format which allows him more time to problem solve but there are now guys in the locker room who see him as an opportunity rather than an immovable obstacle."
Nadal was beaten by Stanislas Wawrinka in the quarter-finals of the Rome Masters, following his defeat by Andy Murray in the Madrid Masters final the week before.
The 28-year-old Spaniard also lost to Djokovic in Monte Carlo and has looked nowhere near the player who has lost only one match at Roland Garros in 10 years of domination.
Djokovic, beaten in last year's final by Nadal, has been in electrifying form and after beating Roger Federer to win the Rome title on Sunday looks to have a golden opportunity to complete his career grand slam.
That favourites tag will bring its own pressures, though, and Nadal, who will be seeded outside the top four, is not the only threat to the Serb's hopes of a first French Open.
"Murray is most certainly a threat to go very deep and, if the draw breaks in his favour, a final is attainable," Courier said.
"Once you reach the final, anything goes -- especially now that Nadal is playing like a mortal on the clay again.
"Kei Nishikori is my second favourite behind Djokovic and I firmly believe he is the second best claycourt player today.
"Also, Nick Krygios has the game, aggressive energy and athletic ability to stir things up the way he did at Wimbledon last year and in Australia in January."
(Editing by Ed Osmond)
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