I would take into account that Nadal has not played a serious tennis match for a very long time and that he has not yet reached the right form.
It’s not the same just to train or play tennis matches regularly.
For him, Roland Garros is certainly the most important tournament this year, and the most important thing for him is to be completely ready for that big tournament.
That's exactly what I was thinking when Nadal lost. Yes he was training while not playing tennis but as I have always said in here and other sporting threads, its nothing compared to match game fitness. I also think that he is getting his body ready for the French Open which would be his main target to get himself 100% match fitness so I think this is just a bump in the road and he'll be ready for the French Open.
That will be his main goal for now, he had a good run in Rome but Schwartzman was nearly flawless. And he said in the interview after that match that he knows what he needs to fix in his game. So I expect him to defend his French Open. At least his body could be like 80%-90% now and I'm sure his mentality will be 110%, and perhaps this is the one that he is referring. But knowing Nadal, he usually adjust and perform very well in extreme pressures.