Speaking on Tuesday's rest day, Valverde told reporters that with most riders having already raced in one Grand Tour this season, general fatigue was a bigger factor than usual.
"It's not the same getting to your hotel in half an hour in a helicopter as taking nearly three hours in a bus like my team did yesterday from the finish to our team hotel," he said.
"When you need to rest, that's a big difference."
Valverde, the runner-up in last year's race, also said he thought Italian favourite Nibali was beginning to look vulnerable.
Nibali's lead of 50 seconds was slashed to just 28 on Monday over veteran American Chris Horner with Movistar rider Valverde 76 seconds behind.
"I would now say that Horner's even stronger than Nibali on the climbs," Valverde said. "And if he hadn't had such a poor (stage 11) time trial, he'd be ahead of Nibali overall."
Victory in the Vuelta would make Nibali, winner of the Giro d'Italia in May, the first rider to win two Grand Tours in a single year since Spaniard Alberto Contador took the Giro and the Tour of Spain in 2008.
Valverde said the 20th stage ascent of the Angliru, reputed to be Spain's toughest single climb, could be where Nibali loses the lead.
"If you've got a minute's advantage going in there as leader, then you should be okay," Valverde said. "But with 28 seconds, that's another story."
Valverde said all the riders were at the limit of their endurance at this stage of the race which ends in Madrid on Sunday.
"That's why using my team mates in Movistar, like I did yesterday, to make the race much harder before the final climb is what will make Nibali crack. As we saw after a hard day, that's when he's vulnerable," he said.
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