The Stampeders (4-1) emerged from their bye week with the expectation Drew Tate had overcome a strained right forearm that sidelined him for two games.
But Tate's arm flared up again after practice earlier this week. Glenn will start against the Riders (5-0) with Bo Levi Mitchell his backup at McMahon Stadium.
Tate left in the fourth quarter of a 36-21 road loss to Saskatchewan on July 6. He was subsequently diagnosed with a strained forearm muscle and hasn't played since although his latest setback has been called an elbow injury.
"He'll probably be not throwing for five, six, seven days," Calgary head coach/GM John Hufnagel said Wednesday. "It depends on how it responds to injections."
Tate's string of injuries extends back to the second game of last season when he separated his shoulder. He returned later in the year but broke a bone in his arm in a playoff game.
The 28-year-old Texan hasn't been able to get a sustained run of starts since Hufnagel declared him the starter prior to last season.
Glenn, 34, did admirable work to get Calgary to last season's Grey Cup game. But the emergence of Mitchell this year has further crowded the quarterback situation.
Mitchell finished a come-from-behind win against Montreal on July 20 when Glenn suffered a right elbow injury.
Mitchell, 23, of Katy, Texas, then went the distance in a 37-24 victory over Winnipeg on July 27. He threw for 376 yards and three TDs and was named the CFL's offensive player of the week.
"You always expect to go out and play great," Mitchell said. "That's always the mindset.
"Sometimes that doesn't happen. Fortunately for me last time it did. Obviously it's going to help your confidence."
Glenn admits Calgary has a unique quarterback situation.
"The biggest thing about the situation is all three guys have played," he acknowledged. "You don't usually get a situation where that has happened.
"It's usually the first-string guy goes down, the second-string guy plays and then it gives the No. 1 guy time to recover and he comes back to his starting spot.
"That No. 3 guy, I've been in that situation before where you really had to, I don't want to say pray because you're praying for somebody else to get hurt, but you really had to go through some stuff to actually get onto the field."
Glenn had four touchdown passes in a 41-38 win and another two in a 17-10 victory against Saskatchewan last season.
Glenn said his arm is healthy and he's looking forward to facing the CFL's only unbeaten team.
"It's a team that beat us last time out and it's the only loss we have this year, so there's a lot of added incentive," he said. "I've got some good relationships with guys on that team, so it would be good to have bragging rights.
"This is a team that nobody has been able to beat so far and they're coming into our house and we have a very good opportunity to beat them. All that plays a part in the preparation and taking advantage of the opportunity."
Despite having had three different starters, Calgary's offence is averaging a solid 32.4 points per game, second only to Saskatchewan (36.6). The unit is also third in TDs with 16, behind the Riders (19) and Toronto Argonauts (18).
"We've managed to win football games," Hufnagel said. "The quarterbacks have taken care of the football, we've been able to get first downs.
"They've done a good job managing the game . . . and the receivers and the offensive line are tuned into the quarterback that's on the field. That's important because everyone has a little bit of a different snap count and timing of the cadence and how the plays are called in the huddle and things like that."
Calgary kicker Rene Paredes has also been automatic, having made a CFL-record 34 straight field goals dating back to last year. He's also leading the league in scoring with 62 points, having accounted for more than a third of 162 points the Stampeders have scored so far.
Tate was scheduled to be examined by doctors Wednesday and wasn't sure if his present situation was related to the forearm strain or something new.
"I don't know really," he said. "I think I got misdiagnosed to be honest with you.
"We'll find out. We're going to find out (Wednesday), I think, what it is."