The Deceuninck–Quick-Step rider struggled during the climbs of the Dolomites and eventually fell from seventh to 19th overall in the General Classification standings.
Evenepoel had endured a positive opening week to the Grand Tour, his first ever, but now finds himself outside GC contention with five stages remaining.
Italy's showpiece event is the 21-year-old's first since fracturing his pelvis last year and, while some believe he could now quit the event, the Belgian confirmed that will not be the case.
"We knew this could happen. I had a total off-day. It's nothing to be ashamed of," Evenepoel said.
"Someone who has only trained for two months cannot expect to be in top condition for a three-week stage (race)."
After previously admitting "the power is getting less and less", Evenepoel insisted he would "gladly" help team leader Joao Almeida, who finished sixth in the Giau stage, push for an improved top-ten overall finish.
WATCH Stage 17 of the Giro d'Italia LIVE, FREE and in HD from 7:50pm (AEST) via SBS On Demand and 8:30pm on SBS VICELAND tomorrow night.