Villa spent most of transfer deadline day batting away offers from Arsenal for the Brazilian defensive midfielder. The last, believed to be around £25million, coming just an hour before the transfer window closed, giving the Midlands club no time to find a replacement.
Luiz, who scored against Arsenal in Wednesday's 2-1 defeat at the Emirates, is out of contract next summer but Gerrard expects him to refocus quickly.
Gerrard said: "I've got confidence he'll understand the situation and the club's decision we didn't want to lose a very important player and a top player so late in the window.
"From a personal point of view, I very much back the club's decision and stance on that. I spoke to Dougie face to face yesterday and he understood where it was at and the position that the club was in.
"He very much respects the decision. Hopefully it will calm down in the hours and days to come and he'll be fine moving forward."
Villa, who sit second bottom of the Premier League, host Manchester City on Saturday after Gerrard added two new players on deadline day.
Midfielder Leander Dendoncker joined from Wolves for £13million and Southampton defender Jan Bednarek signed on loan.
Dendoncker is a doubt with a throat problem while Diego Carlos is out with a long-term Achilles injury.
Bednarek, brought in to help cover for the loss of Carlos, chose Villa over West Ham but the 26-year-old feels it was the right decision.
He said: "It was an emotional day. One minute I am going to West Ham, another minute Villa. There were a lot of conversations with Southampton and all the managers. I ended up at Villa and I'm really happy with the decision. It was a good decision.
"It wasn't easy because if you have another option you have to take the decision like an adult and this is real life.
"It was a tough problem but a good problem. I'm really happy I have these issues in my life, that I have to choose between two Premier League teams."