He said that the loss as 'one of the worst matches' he'd played on clay in 14 years.
In the post-match presentation, Rafa said, "Was a bad feeling. Was a tough day and was a difficult opponent. I am coming from low moments in terms of injuries, and in terms of the mental side have been not easy to accept all the things that have been going on during the last times."
He added, "So just was this kind of day that everything was wrong. I probably played one of the worst matches on clay in 14 years."
Fognini won 6-4, 6-2 to bring to an end Nadal's 18-match winning run in Monte Carlo.
After losing the first set 6-4, Nadal then lost the first five games of the second set with Fognini easing through to the final against another surprise name, Dusan Lajovic of Serbia.
The last man to take a set off Nadal in Monte Carlo was Britain's Kyle Edmund in 2017.
"So when that happens, you have to lose. And today I deserve to lose because I played against a player that was better than me today," he concluded.
Fognini won 6-4, 6-2 to bring to an end Nadal's 18-match winning run in Monte Carlo