He was due to return to full training at the start of this week, but is still experiencing pain.
Corberan, who does welcome back Duane Holmes after a spell in isolation due to Covid issues for Friday's Championship home game with Stoke City, said: "We have the same number of players who we had available (at Reading), but we will add Holmes, but Colwill is continuing with his injury.
"I expect he will be out of the team in the next two weeks as every time he comes onto the pitch, he still has some problems in the back of the knee.
"Now they have decided to give him an injection to progress and ensure he can do the right recovery. We are going to miss him for the next two to three weeks.
"I think it's clear he's been an important player and made an important contribution in attack and defence.
"We didn't have him in the previous games and losing players is never a positive thing.
"We have options to face the games but we never want to lose any players.
"But we know it's part of the season and I believe in the depth of our squad. That's one of our strengths this season compared with last season."
Corberan admits that Town are looking at reinforcements in a couple of areas ahead of Monday's transfer deadline.
One player being linked with the club is ex-Leeds United, Sheffield United and Rotherham United loanee Jamal Blackman, who is a free agent. He has reportedly been training with the club.
Corberan, who is not preparing for any departures, said: "Until the market is closed, we have opportunities to improve our squad.
"We have a couple of needs and that's why there is a gap to improve the squad and as a club, we have the responsibility to look for options and let's see if we find the options and the players to cover the options or we just face the season we have right now."