Successive injuries to running back Brandon Whitaker and to back-up Victor Anderson left an opening that Jennings was only too happy to fill when general manager Jim Popp called in mid-September.
''It was quite a surprise,'' Jennings said Thursday. ''I spoke to Jim Popp during the summer and he told me to stay ready.
''Nothing's guaranteed in life. My mentality was, whoever calls me next, I'm gone. I get tired of sitting around the house.''
Jennings, who turns 27 on Dec. 12, will be the starter when the Alouettes play the Toronto Argonauts in the East Division final on Sunday at Olympic Stadium.
The Yuma, Ariz., native played the final six regular season games, picking up 334 yards on 59 carries for an average of 5.7 yards a run, which is higher than either Whitaker (5.1) or Anderson (4.3) managed.
While Whitaker is a multi-dimensional back who is as dangerous catching the ball out of the backfield as using his slippery speed to elude tackles, Anderson has also shown he can catch and brings a more straight-ahead, punishing style to carrying the ball.
He had been with the Alouettes in 2008 and 2009, but got into only one game before he was let go so that he could sign with the Cleveland Browns of the NFL.
He was released by the Browns the following year, and signed with the New York Jets only to be cut before the 2011 season. He had try-outs with San Francisco and Seattle this year, but didn't catch on.
Then Popp got on the phone and Jennings was a football player again.
''Going back to the time we let him go to the NFL, we always had a gentleman's agreement that he would come back here,'' said Popp. ''He tried to come back a couple of times and we didn't have a job.
''He even offered before camp to come in and play linebacker and on special teams. He probably had that notion because he knew that Avon Cobourne and Diamond Ferri (neither still with Montreal) had gone that route. We just didn't have room, but as soon as the injuries started mounting, we got on the horn and he was here.''
Jennings said being an Alouette is sweeter the second time.
''It's more fun now,'' he said. ''Before I was young and didn't know what to expect.
''I just knew I loved football and wanted to get in there. Now it's just crazy fun being here with guys who love the game like I do.''
The running game is not likely to be a major factor in the East final. The Alouettes and Argonauts were seventh and eighth respectively in rushing yards this season in the eight-team league.
Quarterbacks Anthony Calvillo for Montreal and Ricky Ray of Toronto are more likely to keep the ball in the air. Montreal ranked second and Toronto third in passing yards.
But the ground may be more of an option for the Alouettes, as the Argos' defence, which is tough against the pass, has been vulnerable at times against the run.
Stopping Jennings, a powerful runner in a five-foot-10, 220-pound body, can be challenge.
''When you have a guy like that and you have a lead, you can start pounding them by running the ball and burning the clock,'' said Popp.
An advantage for Jennings is that he had already worked in coach Marc Trestman's offence and knew many players on the team, starting with Calvillo. That helped him blend in almost seamlessly during the season.
''This offence is complicated and he's only been here so long, but he's been able to step in and help us win,'' said Calvillo. ''There's a lot of guys who came through here that I don't remember, but I do remember him just because of how fast and big he was.
''That's what I'm impressed with is his speed for his size.''
Having another experienced player with a positive attitude may also help.
''It's never been about the money for me, it's just that I really love the game,'' added Jennings. ''You get tossed around a couple of times in the NFL and all, and then being away for a handful of months, you start to realize how much you love the game.''