But City veteran Andrew Nabbout will be sidelined with a calf strain.
Auckland's Socceroos bolter Lachlan Brook scored the opener - his sixth goal in nine games - in first-half injury time.
Max Caputo equalised from the penalty spot in the 70th minute before Medin Memeti completed the comeback 11 minutes later - sending City into third place and just four points off leaders Auckland.
"A lot of credit goes to the boys, because they bounced back into that second half and got themselves back into the game, and just weren't content with the draw," Vidmar said.
"They kept fighting and fighting, trying to push for the second one. And then we got that. So that's the mentality that we need."
With Socceroos assistant Hayden Foxe among those watching on, Trewin was hurt in the fourth minute in a sliding challenge from Felipe Gallegos that earned the Chilean a booking.
Trewin played on but was substituted with a left knee concern 20 minutes later.
"He's fine," Vidmar said.
"He got a really heavy knock on his knee, and he was just having some difficulties with it.
"And then he started feeling like a little bit of a numb leg up the thigh. But I just spoke to him then and he said, it's absolutely fine."
Vidmar confirmed Nabbout, who headed down the tunnel in the 58th, had strained his calf.
Memeti's winner came just over a week after it emerged Football Australia were investigating him over allegations he racially abused Brisbane striker Justin Vidic.
Aurelio Vidmar's City chose not to stand down Memeti, 18, while the investigation, which is yet to be completed, is taking place.
"Everything was submitted last week at some time, Wednesday or Thursday, and we haven't heard back yet," Vidmar said.
"So again, still with the FA."
Auckland lost skipper Hiroki Sakai to an apparent right hamstring injury in the 44th minute of their first away defeat this season - but coach Steve Corica said his withdrawal was precautionary.
Corica's side had never previously lost after leading 1-0.
"They were hungrier than us, simple," he said.
New City signing Ryan Teague, who is on loan from Belgian Pro League side K.V. Mechelen as he chases a Socceroos World Cup berth, started and was understandably rusty.
Auckland opened the scoring when Jesse Randall brilliantly slipped through Brook, who nailed his first-time finish.
City appealed for a handball after a skidding Andreas Kuen shot hit Francis De Vries' arm in the 65th minute
After a lengthy stoppage, referee Shane Skinner was called over by VAR Casey Reibelt.
He awarded the penalty and Caputo banged it past Michael Woud to equalise.
"The penalty changed the game," Corica said.
"I've seen them before, not given.
"I just want to know why they actually gave it. Because I thought his hand was down by his side."
The home side snatched three points when skipper Aziz Behich whipped a tantalising cross across the face of goal, where youngster Memeti headed home.
Auckland next host battling Central Coast next Saturday, when City travel to face Macarthur FC.
