This is an interesting final. If Rafa were facing Murray, Federer or even Djokovic, i would expect him to win. Djokovic has given Nadal problems on clay but Rafa had beaten him in a few FO finals before losing to Nole in 2015. This loss should have an asterix next to it as Nadal was struggling back then, nowhere near his best.
With Stan, it's a bit harder to predict. Stan is 3-0 in grand slam finals because when he makes a final, he's very confident and a confident Stan is extremely difficult to beat. When Stan is at his best, he hits with tremendous pace and spin consistently, serves big and moves deceptively well. Stan has an ability to hit winners from anywhere on the court, the way be blew off a dominant Djokovic in 2015 FO final and last year's USO final was scary. He has also taken out Federer and Murray in slams and is just a big obstacle when confident.
Nadal is playing at his best and he's the greatest clay courter of all time but if Stan is ripping groundstrokes and serves, even Nadal will have a lot of problems. Having said this, Nadal's game can present problems for Stan because it's a game of spin and angles, Stan may be forced to stretch for more balls than usual and have to rip winners off heavy balls for hours. Stan matches up well against Djokovic as Novak hits deep but isn't as good as Nadal with angles, it's almost impossible to beat a top form Stan playing Djokovic's game. Murray was able to take Stan out last year by using variety, angles and keeping Stan off guard. Federer does the same when he beats Stan, hitting out with Stan is a no win proposition. Nadal will need to serve big and be masterful with angles, especially the acute short angles he can hit. He will need to use some variety (drop shot, come to net, slice etc..) and he can, Nadal has variety.
One key factor may be the fact that Stan had a tough match vs Murray and given Nadal makes it physical, it could be a problem for Stan. This match is unpredictable, no score will surprise me.