Now, after seeing the spectacular results, the greatest question surrounding Janis is, "Why wasn't he playing earlier?"
Prior to the Packers' loss, Janis had played in just 136 snaps all season. In the last five games, Janis played just 25 snaps.
But when Cobb went down, the Packers had just three healthy receivers, including James Jones and Jared Abbrederis. And Green Bay coach Mike McCarthy had no choice but to turn to Janis.
Janis finished with the third most receiving yards in Packers' playoff history. His two touchdown receptions also tied eight other players for second place in that category.
"Jeff Janis, he's taken advantage of some opportunities and he's just a young man who needs to play," said McCarthy, the man who determines playing time. "He's got a lot of raw ability. He made some huge, huge plays."
Prior to the Packers' loss, Janis had played in just 136 snaps all season. In the last five games, Janis played just 25 snaps.
But when Cobb went down, the Packers had just three healthy receivers, including James Jones and Jared Abbrederis. And Green Bay coach Mike McCarthy had no choice but to turn to Janis.
Janis finished with the third most receiving yards in Packers' playoff history. His two touchdown receptions also tied eight other players for second place in that category.
"Jeff Janis, he's taken advantage of some opportunities and he's just a young man who needs to play," said McCarthy, the man who determines playing time. "He's got a lot of raw ability. He made some huge, huge plays."
After watching Janis feast on Arizona's secondary, it seems almost criminal that he wasn't playing earlier.
Janis (6-3, 219) has almost identical measurables to Jordy Nelson (6-3, 217), who tore his ACL in August. Janis ran the 40-yard dash in 4.42 seconds at the 2014 NFL combine.
Janis has a vertical jump of 371/2 inches, something he showed the world on his game-tying touchdown. And Janis had a 20-yard shuttle time of 3.98 seconds, which was the fourth-fastest among wideouts at the 2014 combine.
After losing Nelson, the Packers' wideout group was one of the slowest in football. Still, Janis couldn't get on the field with any regularity before playing 40 snaps in Arizona — his second most of the year.
"With the type of guys that we have in our room and the caliber of guys that we have in our room, there's no reason why they shouldn't be playing, either," Janis said. "It's just the way the league is. I've got to take my reps where I can get them, whether it's special teams or offense. You've got to make the best of them."
Much was made throughout the year of Janis earning the trust of quarterback Aaron Rodgers. Janis, who played at Saginaw Valley State, wasn't a player that Rodgers ever fully trusted.
But with no other alternatives Saturday, Rodgers had little choice but to feed Janis the ball at times. And the results were sublime.
"I think what happened the other night helps, but it's still something that's going to take a little while," Janis said of his relationship with Rodgers. "It's going to take more practice reps, and more game reps.
"Until that happens, yeah, I think I gained his trust a little bit more. To be a starter would, that takes a whole other level of trust. I'm just going to keep working on that whenever I can, whenever I get reps with him."
Janis showed in Arizona that he could be worthy of far more reps in 2016.
Janis made a terrific route adjustment and beat cornerback Justin Bethel for an 8-yard, third quarter touchdown that gave Green Bay a 13-7 lead.
Later, with the Packers trailing 20-13, they faced a fourth-and-20 from their own 4-yard line with 55 seconds left. Rodgers escaped pressure, rolled to his left, then fired a dart to Janis for 60 yards that kept Green Bay's season alive.
Two plays later, the Packers were at Arizona's 41-yard line, but down to just 5 seconds. The Cardinals rushed six — the opposite approach Detroit took two months ago — and Rodgers escaped to his left.
Rodgers released the ball from Green Bay's 45-yard line a split second before getting leveled and threw it five yards deep in the end zone. The Cardinals had two players waiting for the moon ball — including all-world cornerback Patrick Peterson.
But Janis, who was trailing the play, out-jumped both Arizona players for the ball. And Green Bay's second Hail Mary completion in two months sent the game into overtime.
"Because of the trajectory of the ball and everything, I was like, 'Oh, boy, this is going to be a tough one,' " Janis said. "But I put my head down and ran, turned around and there it was."
Now, here Janis is. And it will be interesting to see what McCarthy and Co. do with him in 2016.
Nelson is expected to be fully recovered by the start of next season. Cobb is the expected No. 2, but he had a disappointing season.
Davante Adams was given every chance to win a starting job, but failed miserably and will have to fight for a roster spot. Second-year man Jared Abbrederis flashed late in the year, while rookie Ty Montgomery had his moments before an ankle injury ruined his season.
Perhaps Janis will be the hidden gem in this group, though.
Janis proved in Arizona that Green Bay's coaching staff erred by not playing him throughout the season. And the 24-year-old Janis hopes that Saturday night was just the start of something special.
"I think the biggest thing is it tells me to myself that I can play," Janis said. "When you have a confident player, I think that helps.
If you haven't yet added him to your bench, get him. 
