Sky's Chris Froome leads the race by 27 seconds on both Rigoberto Urán (Cannondale-Drapac) and Romain Bardet (Ag2r La Mondiale). Aru, who lost 31 seconds on stage 17, trails at 53 seconds.
Only four stages are left to race, including the summit finish to the Col d'Izoard on Thursday and the stage 20 time trial on Saturday in Froome's favour.
"From a time trialling point of view, he's taking a step back and he's going to have to go long if he wants any chance of winning the race," Sky boss David Brailsford told Cycling Weekly.
"It's a big ask for him to gain that back and you can expect a long-range attack if he really wants to ride to win."
Aru won the 2015 Vuelta a España when he and his Astana team-mates overhauled Tom Dumoulin on the final mountain day.
However, to have any chance of winning the 2017 Tour de France, Aru not only has to take the race lead tomorrow over Froome but do so with around one-minute's advantage.
The final 22.5-kilometre time trial tilts the Tour in favour of Froome.
"If Chris has a 30-second advantage going into the time trial, then we'd take that," added Brailsford.
Sky dominated the opening time trial by winning with Geraint Thomas and putting four of its nine men in the top 10. Froome placed sixth.
"It's all set up not for a great day. A race to top of it is a lot different to a race to the top with a long descent [like the Galibier to Serre-Chevalier], so the competition of the race is a lot different."
The Tour has never finished at the Col d'Izoard, where Louison Bobet and Fausto Coppi once ruled. Froome, however, will be ready as he has previewed the famous pass at 2360 metres.
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