Brandon Bridge of Mississauga, Ont., the first Canadian to play quarterback for Montreal since Gerry Datillio in 1985, took over midway through the third quarter after LeFevour was injured on only his second play from scrimmage.
Ernest Jackson caught a touchdown pass for Ottawa, which also had a Delbert Alvarado field goal, a safety and a two-point conversion by quarterback Henry Burris.
Tyrell Sutton and Jean-Cristophe Beaulieu had TDs and rookie Boris Bede had a field goal for Montreal, which led 13-5 at halftime despite Crompton going only 5 for 17 for 51 yards.
Sutton scored on the opening series as he carried 27 yards to set up his own five-yard TD run. Bede missed the 32-yard convert, mainly because the holder, Bridge, had the ball facing the wrong way.
Burris marched the Redblacks back to the Montreal 29, but Alvarado's field goal attempt was blocked by John Bowman and recovered by Allan-Michael Cash.
Cash picked off a pass in the second quarter, only to have Ottawa's Moton Hopkins intercept a Crompton pass on the next play. That set up Alvarado's 40-yard field goal at 4:11.
The Redblacks picked up a safety, but Jonathan Hefney picked off a Burris pass and, six plays later, Crompton hit Beaulieu with a four-yard TD pass at 9:18.
Ottawa looked ready to answer back when Burris threw his third interception of the half to Billy Parker.
Burris made up for it on the opening drive of the second half, an 11-play drive kept alive by a fake punt and a fake field goal attempt. It ended with a 13-yard TD pass to Jackson and Burris made the two-point convert to tie the game 13-13.
LeFevour went in for Crompton, who had a shoulder injury, to start the second half and was injured on a high hit from Adbul Kanneh. In went the rookie Bridge, who found S.J. Green with his first pass and got the Alouettes into position for Bede's 37-yard boot.
Bridge was picked off by Jovon Johnson and Ottawa marched back for Johnson's six-yard TD run 4:40 into the fourth.
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