Henne, who'll make his third start of 2013, completed 7-of-13 attempts in the relief role and has connected on 53-of-95 throws for the season, compiling 601 yards, two TDs and two interceptions.
Gabbert is last in the league with a 36.0 passer rating and is 5-22 in his career as a starter. Henne was 13-18 in three years of starts with the Miami Dolphins and was 1-5 as a starter for an injured Gabbert last season.
"With (playing quarterback) comes the responsibility of being a leader," Jaguars coach Gus Bradley said. "Our guys are working toward that, but they need to step it up. We feel strongly about both of them, we're going to challenge both of them and they're going to need to step up."
Blackmon's TD from Gabbert was his first of the season after a four-game suspension to begin the year. He caught five balls for 136 yards and can give Henne a second viable option beyond Cecil Shorts, who has 31 catches for 411 yards through five games.
On the ground, Jacksonville is 30th of 32 teams with an anemic 58.8-yard average per game. Veteran Maurice Jones-Drew is averaging just 2.8 yards per carry. And the offensive line took a hit when left tackle Luke Joeckel broke his ankle against the Rams.
Denver is tops in the league against the run, allowing 69.6 yards per game on 21.2 carries per week, which is second-lowest in the NFL. The more the Jaguars can hold onto the ball against whatever shortcomings the Broncos' defense has, the less time they'll spend contending with Manning.
"He's the best that I've ever encountered," said Bradley, an assistant in Seattle for four seasons before taking the Jacksonville job. "How he manages the game, it appears like he never puts the team into a bad call. He knows everything about you on defense. He studies. He controls the whole offense. He's very talented."
WHAT TO WATCH FOR
Any measure of success the Jaguars have had on offense this season has gone through Shorts, or his recently returned colleague, Blackmon. Having both of them together for the second time this season can't help by help Henne, who's not been able to get much from a running game.
The Broncos could have both starting cornerbacks, Bailey and Dominique Rodgers-Cromartie, playing together for the first time in 2013, which also helps what's been one of the league's leakiest pass defenses through five games.
As for the Broncos' offense, well, good luck Jacksonville. Manning has played the best football of his career through the first five games, which is saying a lot considering the success he's had since 1998. But seriously, a Jaguars defense that has surrendered more than 400 total yards this season to youngsters Andrew Luck and Russell Wilson seems impossibly overmatched with Manning and his band of scoreboard-torchers.
It really doesn't seem fair, right? You get the feeling in Manning wanted to score 100 points this weekend, it wouldn't be too far-fetched to think he actually could.
Of course he won't, and he'll more likely be on the bench in the third quarter, but it'll probably look pretty ugly as long as it lasts. A moral victory for Jacksonville? It says here Denver won't cover the spread.
Sports Network predicted outcome: Broncos 42, Jaguars 19