Aru lost 41 seconds on stage 11 when there was a late surge and he was not close enough to the front. He now lies 13th, 1:49 behind race leader Simon Yates (Mitchelton-Scott).
Although not a disaster, the UAE Team Emirates leader has barely impacted the Vuelta so far, even compared with other contenders behind him in the standings like Wilco Kelderman (Team Giant), 14th, Michal Kwiatkowski (Team Sky), 15th, and Thibaut Pinot (Groupama-FDJ), 16th. Even his compatriot Vincenzo Nibali (Bahrain-Merida) appears to be on the way up after an uneven start, whilst Aru is flat-lining at best, sinking at worst.
Where to go from here? Aru seems anything but certain, as the race heads back into the mountains. And come the Camperona on stage 13, the team and he are hoping for the best, but braced for a tough ride.
"I wouldn't say he's doubtful, but he's certainly lacking a little bit to be up there with the best. He knows that, we talk about it, it's not a secret," UAE Team Emirates director sportif Philippe Mauduit told Cyclingnews.
"The thing is he had a tough start to the season, and he's having to fight hard to hold on. That's something that happens to every champion at some point in their career, and it's happening to him now.
"We're not ruling anything in or out for now, we're taking it all on the day by day, and with each stage result, we'll sit down and work out what happens the day after."
Aru himself is equally realistic about his chances.
"To be honest, I'm a bit down on GC and when it comes to the last few kilometres and changes of pace, the guys going well are up there," the Italian said.
"These first 10 days - I've not had that. The only thing to do is to take it day by day and see what happens. There are still 10 days to go and in these difficult moments, it's hard to see what the objectives are because that is the situation.
"We'll have to see what happens. These days are proving very complicated for me. I'm not getting dropped at the start of a stage, but I'm lacking that extra bit in the last few kilometres."
He knows only too well, too, what is coming up. "I did the Camperona and the Lagos de Covadonga in 2014" - when he finished fifth overall. "They're both really hard, and very demanding."
Mauduit is adamant it is not a lack of form, because Aru, he says, has trained hard and is in good shape. "But it's just the last few percentage points that [he's missing which] make the difference."
He does not think the switch in the weather on the race to much colder temperatures and rain, which has suddenly kicked in at the Vuelta, will make a difference, either for better or worse. "Fabio's always done well in bad and good conditions," he said. "It's not about that."
Either way, the three summit finish stages in Asturias and Castilla are so hard they will likely definitively set Aru on one path or another in the 2018 Vuelta.
For now, the Italian is crossing his fingers that path does not take him downhill towards another disappointing result in what is proving to be an exceptionally difficult year.