Brewster, 40, had a previous 14-month spell as player-boss at the Caledonian Stadium before leaving to take over at Dundee United in January 2006.
On Sunday, Caley football chief Graham Bennett said: "The deal has been done in principle and there will be a press conference at 11.00am tomorrow.
"There were three or four different people in the frame and we spoke to a few of them.
"I am glad it's been concluded today and I look forward to tomorrow's press conference.
"At the end of the day there are no guarantees in football, even if you have Jose Mourinho, but we are just glad we have our man.
"The situation with Charlie Christie leaving has put everyone one on a downer and we've not had a great start to the season but we've got to crack on."