Bo Levi Mitchell suffered a sprained left ankle and stretched ligament in his knee late in Saturday's 40-33 win over the visiting Toronto Argonauts. Stampeder coach John Hufnagel says Mitchell's status is "week to week."
Drew Tate will get his first start of the season next Sunday in Montreal against the Alouettes. Calgary (10-1) is looking for a sixth straight win.
Tate's been on the field in all 11 games this season for short yardage situations. He's scored four touchdowns.
The 29-year-old Texan was Calgary's starter out of training camp in both 2013 and 2012, but a run of injuries limited him to 153 completions those years. Tate is 5-0 when he's played an entire game.
Mitchell edged out Tate in what Hufnagel called a "photo-finish" to get the ball this year.
Mitchell was 9-1 and led the league in quarterback efficiency heading into Saturday's game. He ranks second in touchdown passes thrown with 17 behind Ricky Ray's 20 with Toronto.
Mitchell left Saturday's game with just over six minutes to play and Calgary trailing by eight points. Tate continued Calgary's rally with a touchdown throw to Jabari Arthur and a two-point convert pass to Jon Cornish to tie the game.
A short pass to Cornish for a 30-yard scoring run capped a comeback victory.
"I have been in this situation before, so I think it helps definitely with that, knowing I don't have to do too much," Tate said Sunday. "We have good players across the board so just do my job to get them the ball and let them do what they do.
"I love my teammates a lot, so to get the opportunity to be accountable to them is just awesome. I've been here for awhile and haven't had much opportunity to do that, so here is one. I just want to take advantage of it."
Winnipeg Blue Bombers quarterback Drew Willy left Saturday's loss to the B.C. Lions in the first half with what appeared to be a shoulder injury.
Saskatchewan Roughriders veteran Darian Durant is out for the remainder of the regular season because of an elbow injury suffered last week.
Lions quarterback Travis Lulay dislocated his shoulder last week and is on their six-game injured list. The Lions, however, have a No. 2 with even more experience than Tate in Kevin Glenn.
Glenn went 20-8 with Calgary before he was claimed in December's expansion draft by Ottawa and subsequently traded to the Lions.
The Stampeders rallied from a 19-point halftime deficit with a tremendous second half Saturday, but paid a heavy price as four of five players who didn't finish the game were starters at their position.
Defensive end Charleston Hughes sprained his foot and his backup Ben D'Aguilar hurt his knee. Both players are week-to-week.
Defensive tackle Demonte Bolden and receiver Marquay McDaniel sustained broken bones in the leg and hand respectively and will go on the six-game injured list.
The Stampeders are accustomed to change behind centre as Tate, Mitchell and Glenn all started games for Calgary last season en route to 14-4 record. Despite their decimated defensive line, Calgary didn't allow a touchdown in Saturday's second half.
"This team is built to overcome adversity," Cornish said. "We're equipped with personnel depth that I think is unmatched in the CFL.
"It's really sad to see Bo go down, but having Drew there to finish out the game, that's huge. It means there's no real falloff."
Mitchell was tackled on a four-yard run by Toronto's Shea Emry and went down awkwardly on his left leg. Mitchell, Lulay, Durant and Willy were all injured on football plays with no flags thrown.
"Probably bad luck," Cornish said. "People want to see these events happening close together and put them together somehow, but it really is just coincidence.
"As the science behind strength and conditioning gets better too, you're going to see bigger and bigger hits. Especially when someone's not necessarily prepared to take the hits, it's pretty extreme contact. You don't want to see people taking these hits, but I guess it's just part of the game right now."
Mitchell was not at McMahon Stadium for Calgary's post-game rundown Sunday.
"Feeling a lot better this morning! Ready to get strength back this morning, so that I can be back soon," Mitchell said on Twitter.
Tate's last start was July 5, 2013, when a throwing arm injury sidelined him in the fourth quarter against Saskatchewan.
Even though he got back into games late last season, Tate says it was only three weeks ago that his arm felt pre-injury strong. Being Calgary's No. 2 behind Mitchell gave him more recovery time.
"That was somewhat of a blessing in disguise I really think," Tate said. "I think it started feeling good, I could really tell the difference was the Ottawa game, the second Ottawa game out there I could really tell in warmups that the ball was just coming off better and the spin was tighter.
"It's helped me personally and hopefully now I can just stay healthy and win some games."
Calgary's No. 3 is Bryant Moniz out of the University of Hawaii. Hufnagel is expected to sign another quarterback as insurance.