Socceroos centre-back Trewin on Wednesday sealed his move to Major League Soccer sister club New York City FC.
Trewin, 24, wasn't present for last week's loss to the Bulls, City's heaviest under championship-winning coach Aurelio Vidmar.
"We had a really robust discussion yesterday," Vidmar said.
"To be honest, the effort and everything was there, but we really got punished with our simple mistakes, our basic mistakes, and if you have a look at all the goals, they were just very simple mistakes, which is very uncharacteristic for us.
"So yep, absolutely got to be better, but we're resolute as ever. It was a really disappointing result.
"But we can't sit there and cry about it. We've got just a couple of things, got to make ourselves better, always looking and wanting to improve.
"We are in a really good headspace, and sometimes you get a bit of a jolt when it's a heavy defeat like that, but we've got to just keep coming back and keep showing ourselves."
He praised World Cup hopeful and championship winner Trewin.
"As disappointed as I am that we're losing him - because I would have preferred to lose him at the end of the season - I'm pleased for him," Vidmar said.
"If he continues his form, he's able to play on a weekly basis, I think he's a shoo-in (for the World Cup)."
Defensive signing Harrison Delbridge won't be available for Friday's away trip to Wellington and eighth-placed City are unsure if recruit Daniel Arzani will play.
City players and staff are reeling after the death of former ALM and ALW coach Rado Vidosic, following a short battle with cancer.
Vidmar completed his professional licence alongside the popular Australian football stalwart and later replaced him as City coach.
"Devastating news. A guy that's done an enormous amount of great work in football in this country for many, many years and also served this club with high distinction in the women's game, the academy space, and also the first team here at City," Vidmar said.
"It can be a harsh environment in football sometimes but he always was the same person, a true gentleman right through his time as a football coach.
"So it was really devastating news for everyone and he's going to be missed for sure."
