The Melbourne-born driver will start 16th in Sunday's race, with British teammate Lando Norris set to line up 13th.
McLaren have yet to score a point in two races so far this season.
"We adjusted the front wing a little bit for the last run and maybe it wasn’t quite the best for the track conditions improving," Piastri told Sky Sports television.
"So yeah, I guess a little bit of inexperience on my side potentially. But it wasn’t a big issue, it was only maybe a small tweak...it wouldn’t have cost us too much."
Piastri is the only Australian on the grid, with experienced multiple race winner Daniel Ricciardo -- his predecessor at McLaren -- taking a year out as Red Bull reserve while he considers his future.
With Sergio Perez bringing out red flags in the first session after beaching his Red Bull in the gravel, Piastri eventually found himself battling traffic as he tried to set a good enough lap.
"All weekend it’s been difficult to get the first lap going with the tyres, just because it’s so cold," he said. "I think what we did was fine, just not quite quick enough."
Piastri had already been reprimanded, his first in Formula One, after final practice for a breach of the practice start procedures.
"The driver admitted that he had lost concentration and was preparing to do a practice start but attempted it a lap too early, having failed to take the chequered flag before doing so," stewards noted.
"After he slowed down considerably, the team warned him to complete the lap before attempting a practice start and therefore he did not go through the full practice start procedure.
"The driver candidly admitted his mistake and recognised that this could have resulted in a dangerous situation on the track."
