With Nadal now likely to miss 2 months at least (same injury as in New York and he was out 2 months before he then needed an operation on a different ailment) thiem is probably favourite for the French Open given Djokovic will feel pressure going for undisputed GOAT status if he were to hold all 4 Majors at once again for 2nd time.
Djokovic is getting quite old for clay, but his seeding will help him immensely this year..... Let's check the potential seeding. If Thiem can hold off Nishikori in Miami (will be 165 point gap in live rankings Monday, 565 if Thiem wins IW), he's well setup for garnering 4th seed at RG unless Federer decides to play Monte Carlo (Thiem likely to get 360 points in MC assuming he gets to SF.) Nishikori is set to play Barcelon so he'll skip Monte Carlo. #4 at the French for Thiem seems likely if he starts out clay season at or near 4 (5 behind Federer). If Thiem is 5-8 seed his chances are much lower as Zverev/Nadal or Zverev/Djoko may await him for QF/SF with either Djoko or Nadal in Final.
Let's assume Thiem gets 4th seed for RG. That leaves Fed, Nishi, Anderson, and likely Tsitsipas 5-8. Sneaksipas probably very dangerous for Thiem and Djokovic, but not Nadal. R16 for 5-8 will be with 9-12 and here we likely have Khachanov, Coric, Raonic, and maybe Cilic. Khachanov and Coric have the potential to cause a lot of trouble so they could come through to QFs with the top seeds. I'd pencil in Basilashvilli, Cecchinato, Medvedev (hardly defending anything on clay), and lets say Wawrinka for 13-16. Medvedev is not threat on clay. Wawrinka and Basilashvilli might be extremely dangerous for the top seeds R16. Hard to predict beyond this, but Shapo, Demon, Tiafoe, possibly Edmund, Goffin, and Auger-Aliassime will be minor threats (FAA with solid chance of seeding by RG).
Djokovic hasn't really proven himself on clay the last few years and he's no spring chicken so very long in tooth for clay. Also factor in the possibility of Wawrinka or Cecchinato R16 and his draw could be problematic. Thiem and Nadal have little to fear from R16 group. If Zverev broke through at RG he'd be a rough SF as well and Thiem assuredly. Tsitsipas dangerous.
Nadal only has to fear Djokovic, but that can only happen in final. Zverev almost took him out in Rome and Thiem would spell trouble in wet or cold conditions where the bounce is not terribly high. Coric in fantastic form about the only QF that might put some pressure on Nadal.
Thiem has a good advantage at 4 as the big serving and dangerous Zverev only possible in final. Nadal SF could easily be the end of the road. Tsitsipas and Coric in great form would be huge threat to Thiem. Thiem obviously better off in Djokovic's half and let's leave it at that.
Zverev's issue is players that can bleed him. These are typically mobile players who can engage him in long rallies. If the last two years are any indication he'll come into RG with a head of steam. 7th heaven for Zverev if he draws a bunch of big servers who won't bleed him. The weakest of the 4, but would be no joke if he came into latter stages having only faced a few 4 set matches. Still, likely to get bled or knocked out early at RG and be no threat to anyone in the latter stages.
Looking at the draw possibilities Thiem is a clear #2 at RG unless Djokovic gets another nice slam draw like at 2018 Wimby and US Open.
We have not statistical baseline for Thiem coming into clay this year given his sickness and injury plagued start to 2019. His IW run has been beyond impressive with very solid stats.
@Gary Duane and I were secretly lusting over Thiem's hard court stats and potential on serve at this time last year and then disaster struck and Thiem fractured his ankle. This completely derailed his start to the the clay season from lack of play and Thiem did not begin to play reasonably well until late in Lyon even playing two matches in the same day right before the final. This gave him the play and momentum for RG, but even with that his stats for RG were not as good as 2017 despite making the Final. Watching his movement (viper like quickness some of the time vs Raonic which has his backhand really clicking), it appears to my eyes that Thiem is set to exceed even my own lofty dreams. Break it down:
1. Thiem's serve and forehand go up and down in level together. First set against Raonic he poured in over 90% of his first serves and won over 90% of those points with 0 UEs (finished match with something ridiculous... 9 UEs and two were DFs). Average first serve speed was 110 mph. Forehand was a rock and this all implies that Thiem's hip/back issues that have impacted his game are non-existent at the moment. Thiem is healthy.
2. The big serve game Gary and I saw forming at the start of 2018 has arrived and to add insult to injury to the field Thiem seems at a level late at IW much like his big Acapulco run where he had something like 22 Aces and near 80% serving in the final and much the same for the whole week. Thiem's serve game has arrived and its incredible. It will be lethal on clay where his first kicker will be highly effective (by the way this serve has been changed due to health concerns and still pales to what he had going with the "Super Kicker" Acapulco 2016.)
3. Thiem appears to be a late bloomer. His movement against Raonic is far better than coming into this period last year which is amazing given he's 25 going on 26 this Fall.
4. The Thiem backhand is the best he's had to date due to the viper like movement giving him more time. Not going to be pretty for opponents on clay as Thiem can hit through even Nadal with his big backhand.
5. With this Raonic victory Thiem has clearly figured out to handle giant servers like Raonic, Karlovic, and Anderson back at US Open. This has been a real weakness for Thiem who has an excellent first return when he's up in the box blocking, but it does not work against the giant high velocity servers. He's able to stay way back now and flag back enough returns to put these servers into pressure ever few games. Far away from the likes of Murray or Djokovic particularly in recent times (Nole's first return had been declining, but now he's handling big serves with aplomb.) But for Thiem this is enough and now that he has great movement and a big serve game he's able to dominate baseline rallies with these players. Won 65 baseline rallies to like 20 with Raonic earlier today.
6. Thiem's patching of his weaknesses means he's going to be going deep with regularity and may even be a threat for number one over the next few years before the younger NextGen wave comes to fruition.
7. Thiem's stamina continues to improve nicely and no doubt he's capable in this area now for any slam, something not the case in 2017 and we started to see with his epic QF with Nadal at the 2018 US Open.
8. Thiem's mysterious 2nd serve woes of 2017 into 2018 really held him back and they now seem firmly in his rear view mirror. Even if still a bit of an issue it is a minor concern if Thiem is able to serve 70-80% first serves in latter stages of events.
The only weakness for Thiem on clay is high bouncing conditions at RG allowing Nadal to continue his abuse at that event. If the improved movement allows Thiem to blunt this then he'll be a big threat to Nadal and perhaps able to do physical damage to Nadal like at 2018 US Open.
Overall, Thiem for me after review is is a strong #2 at RG and the only thing really holding him back on clay for the time being appears to be Nadal.