Nibali was speaking on Sunday, the day after failing to stay with the overall contenders on stage 8 to Saint-Etienne, and so losing any chance of overall victory at this year's Tour.
The Italian was distanced on the last climb of the stage and sat up, losing four minutes. He lost a further two-and-a-half minutes on stage 9 to Brioude on Sunday, slipping to 30th overall, 8:56 down on current race leader Julian Alaphilippe (Deceuninck-QuickStep).
The Sicilian finished second at the Giro d'Italia behind Richard Carapaz (Movistar) in early June. He had always said he would target stage victories at the Tour de France if he was unable to be competitive against riders who have specifically prepared for the sport's biggest Grand Tour, but many fans were hoping he could repeat his 2014 Tour success – especially in the absence of the injured Chris Froome and Tom Dumoulin.
Nibali's coach Paolo Slongo suggested that the Bahrain-Merida leader had given up mentally more than physically. That sparked criticism on social media, and Nibali responded directly to one post that suggested he should have dug deeper, saying that people don't just give up in real life.
"That's not my explanation, and I don't think I need to give one," Nibali hit back via Twitter. "I do my job and my sport as best I can because I love it. If I get dropped, it's because I couldn't stay on."
Nibali responded further on Sunday, clearly irritated by what he described as people who "just watch the Tour on TV, drink Coca-Cola, eat popcorn and criticise people on social media". There is also some tension within Bahrain-Merida, with Nibali set to join Trek-Segafredo in 2020.
"I can understand that the tifosi always want to see some spectacular racing from us, but I hope that they can understand that it's not always easy. I tried my hardest, but when the legs say no, there's nothing you can do," he told Cyclingnews and La Gazzetta dello Sport.
"We have good moments when we win, but there are often difficult moments. I've learned to accept both, because when we suffer, I think we show our human face.
"The level is very high at the Tour de France, so you have to be at 100 per cent. Riders like Thibaut Pinot [Groupama-FDJ] are peaking now after a very quiet first part of the season. I'm not at my best; I'm stuck in the middle. I have the endurance, but I lack the explosiveness on the climbs to make the big efforts.
"Doing the Giro-Tour double is very difficult to handle. It all depends on your recovery and your muscle type, but I've always struggled. That's why I never promised to target the GC at the Tour this year. I'm much better at targeting the Giro and the Vuelta a España in the same season, as my results show."\
Nibali has already said he feels stuck between a rock and a hard place, with fans and his team expecting so much of him, even when he's not at his best. He admitted he will probably have to lose even more time if the GC contenders are to let him chase stage victories in the Pyrenees and Alps, but seemed pessimistic about his chances.
"Winning a stage would be good. My form's not bad, but I'm lacking that explosiveness for the hard, short climbs. It's a sign that the fatigue from the Giro d'Italia is still there. I'm not in bad shape, but I'm missing the difference between winning and losing, and I'm much more likely to lose than win," he said.