The 28-year-old captain is yet to play for Monk, having been missing with a hamstring injury since August's defeat at Preston North End.
Monk has only lost one of his eight league matches at the helm, winning four and guiding his team to fifth in the table, one point behind second-placed Preston.
The centre-back was back in training on Wednesday but it remains to be seen if he will be rushed back for Saturday's trip to Blackburn Rovers, particularly with the stand-ins performing so well
Dominic Iorfa has deputised well for Lees and on Saturday we was partnered by central midfielder Sam Hutchinson, who dropped into the back four after Julian Borner pulled up in the warm-up. It is hoped the German will be fit for this weekend's trip to Ewood Park.
That Wednesday have not allowed the loss of Lees, or the early-campaign disruption of a caretaker manager after Steve Bruce left for Newcastle United midway through pre-season is great credit to the players.
Monk thinks the mentality has been a big part of that, and wants them to continue to develop it over the course of the season.
“There is so much football to be played but what we need to concentrate on is to keep performing, keep growing," he told his club's official website.
“The main part has been that mentality and determination and I see it in every game, every single training session.
“I’ve told the players if they can keep that, especially in this league, it can take you a long way.
“Then if you add the quality we have and take the things we are trying to improve – that’s how you raise standards and raise performances and that’s what we have to concentrate on.”