Gus sustained the knee injury at Gillingham on Saturday and has undergone scans that have not revealed good news in terms of recovery time.
"He damaged his knee, won't start the season and could be out for some time - we don't know quite how long," reported Glenn.
"It's a big blow for Gus, he did not have one day last season in the treatment room. He's had a scan and it's not going to be anything this side of three months.
"It will certainly be that or a little bit more. It is a blow to us."
Wednesday evening's encounter with Crystal Palace at Selhurst Park could also have come at a price. Dean Richards left the field of play with a head injury, while there are fears that Milenko Acimovic has damaged a hamstring.
"Deano's got a bit of concussion - so it might knock a bit of sense into the big fella!
"Milenko is a little bit more serious, it was a contraction of the hamstring. What with losing Gus the last thing I wanted was for anything to happen to Milenko. We're hoping it's a contraction rather than a pull and we got him off pretty quickly to protect him.
"This is the horrible feeling for managers in pre-season, you have to play the games for the match practice. But, at the end of the day, you can end up getting injuries - we saw that Schmeichel may have done a cartilage.
"It happens to every club, not just us, and we've got to battle on and hope Milenko is not long-term."
Glenn has suffered a major pre-season setback with the news that Gus Poyet could be out of action for in excess of three months.