The former Sheffield Wednesday captain, likely to be a big player for Town this season, joined the club on August 1 on an initial two-year deal. Town have an option to extend the contract for another season until the end of 2023-24.
The 30-year-old has not played since late April after being stretchered off with an ankle injury in the Owls' game at Middlesbrough - forcing him to miss Wednesday's final two matches of the 2020-21 campaign.
Lees subsequently underwent ankle surgery and is now working on his fitness ahead of first-team involvement, with Corberan confirming that the experienced centre-half is ahead of schedule.
Corberan, hoping to be present in the dug-out at Saturday's Championship home game with Fulham - following his time in isolation after contracting Covid - said: "We know that Lees is a player who had one injury from the previous year and surgery on the ankle. "But his level of work in that time was excellent. I received a player in very good physical conditions and I don’t have to talk about the football condition and he is very strong.
“He is close to two weeks of work and he is doing a lot of work by himself and I am thinking he is closer to the team than I expected when we signed him."