Shell V-Power Racing chief Ryan Story laments the "unforced errors" that cost Fabian Coulthard a chance to fight for the Superloop Adelaide 500 podium on Sunday.
After enduring a disappointing 2018 as team-mate Scott McLaughlin won the title, Coulthard enjoyed a return to form in the season opener.
He was third, fifth and first in the opening three practice sessions, qualified third provisionally and then claimed ARMOR ALL Pole on Saturday for the Ford Mustang's debut.
McLaughlin won that race, as Coulthard wound up sixth after serving a penalty for moving out of his starting box before the lights went out.
On Sunday, Coulthard was set to start third but was excluded from the ARMOR ALL Top 10 Shootout due to a pitlane infringement and instead lined up 10th.
His progress in the race was halted by having to pit on consecutive laps during the Safety Car period, after a refuelling error when he came in the first time.
That meant Coulthard took the restart in 21st, and after a tangle with Garry Jacobson and subsequent drive-through he finished 20th, 63 seconds down on dual winner McLaughlin.
Coulthard's weekend went downhill after pole on Saturday
"It was a bit of a one-up, one-down day for us and somewhat bittersweet," Story said of the Sunday.
"Fantastic to win the Superloop Adelaide 500 and for Scotty to take away two wins, but over the course of the weekend we had two really, really fast racecars.
"[On Sunday] we made a couple of unforced errors on Fabian's side and it really hurt him.
"We are bitterly disappointed for him, he certainly deserves better and we'll be sure to regroup and make sure we don't make any of those errors again."
DJR Team Penske led the development of the new Mustang, the first two-door Supercars, with Ford Performance in the US.
Even with the setbacks, Coulthard still leaves Adelaide nine places higher in the championship than in 2018, 11th against 20th.
He only managed qualifying results of 18th and 12th at the street circuit 12 months earlier, but found form even if the way Sunday unravelled left him "a little bit low".
"Obviously we had really good pace, and got the first pole for the Mustang which was really cool," Coulthard said.
"Scotty then backed that up with two wins, so it's nice that our team got the first poles and the first race wins for Mustang.
"We've spent a lot of time developing this car, starting from scratch in CAD to what you see on the track today and a successful racecar.
"For me, we were fast, but some things were out of my control and we probably should have been on the podium quite easily [on Sunday].
"But with what happened in the pits, it just wasn't our day.
"On the upside, we're heading to the Grand Prix and we normally go well there.
"We've got a really good buggy, so I'm looking forward to it."