It was 2015-2016. Expectations for Nadal were a lot lower in those years, even from his fans. Berdych beat Nadal at the 2015 Australian Open 6-2, 6-0, 7-6, having not beaten Nadal since 2006 prior to that. Fognini beat Nadal on clay multiple times in 2015 in a troll like manner, Nadal played probably his worst match off the ground in the 2015 Madrid final against Murray, and Djokovic beat Nadal 7-5, 6-3, 6-1 in the 2015 French Open quarter final. Nadal beat Fognini in a bonkers match in the 2015 Hamburg final, 7-5, 7-5, where there were 12 breaks of serve, and it felt like a big win at the time. Then came an even more bonkers match between Nadal and Fognini at the 2015 US Open, where Nadal led by 2 sets a break, and again there were a tonne of service breaks, and this time Fognini won. I think the hearts of all Nadal fans sank at that moment.
Ironically, Nadal's best and most consistent form of 2015 was late in the year, post-US Open, although he didn't win a tournament at that time of the year.
2016 in some ways was even worse than 2015, with a first round loss at the Australian Open, where Verdasco hit 90+ winners against Nadal again (like in 2009) and this time beat Nadal in 5 sets, and a fourth round loss to Pouille at the US Open, and had to withdraw with a wrist injury during the French Open and couldn't play Wimbledon. In the majors, Nadal was worse in 2016 than 2015. However, Nadal's form in winning Monte Carlo and Barcelona in 2016 was probably his highest point of the entire 2015-2016 period. 2017 was a big revival year for Nadal that seemed a long way off in 2016. In 2017, it was suddenly Nadal and Federer all the rage again, when 2016 had been about Murray and Djokovic.