The first half of that contingency plan is obvious to anyone who watched the Broncos' first three regular-season games this year:
When Trevathan succumbed again to a fracture just above his knee on the second play of Sunday's game, Marshall knew the drill: he changed helmets to the one equipped with a radio receiver to hear Defensive Coordinator Jack Del Rio's calls, and went to work.
"Our defense, we never flinch," said Marshall. "So I just came in and I'm like, 'Let's go, man.' We live for this."
Added defensive tackle
But the big change came when the Broncos used their nickel alignment. In Weeks 1-3, starting middle linebacker
"He's a smart kid that's willing to learn and study, to do the things necessary to succeed," said Head Coach John Fox.
"I think just working hard, busting my butt to get to the ball, listening to the coaches, listening to my assignments and what I had to do, and just being as fast as I can with that," Nelson added. "That's what allowed them to trust me to be able to do that. I just tried to remain consistent, especially on special teams, and those things just all added up to now."
Nelson's teammates noticed.
"He'd been working his tail off," said Marshall.
Added Knighton: "He prepares well during the week. He asks a lot of questions. He's a knowledgeable guy. He's in the game. I have faith in him and I'm not worried about him showing up."
But Nelson still has much to learn. On the first play of the Broncos' second defensive series after halftime, Marshall had to burn a timeout because Nelson did not take field, leaving the Broncos with 10 men.
"I guess he thought it was a different package," said Marshall. "So I had to call timeout, because it was an empty set; they were going to pass the ball, and if he wasn't in there, it might have been a touchdown."
"I came off and I was talking to the special-teams (Coordinator Jeff Rodgers) and I just lost track of what personnel was out there," Nelson said. "That's on me. I take responsibility for that."
"I felt like I could always do better," Nelson said. "I felt like there's a lot of improvement, there's a lot of work to be done as far as the individual spot and the position that I'm in, but I feel like I did well."
Nelson was stellar against the run and solid in coverage. When he dropped in coverage, he kept an eye on Jets quarterback Geno Smith to watch for potential scrambles, but he didn't lose track of offensive targets who ran under or near his zone.
"And our defense is complex too, so for him to come in and play on the road like that and kind of just get thrown in, I've got to tip my hat to him," said Harris. "We need him to continue to improve because it's certainly going to get tougher. We're going to face better offenses than that so we're going to need Corey to step up big now."
With Trevathan once again sidelined, urgency is attached to the growth of Marshall and Nelson. But the alarm sirens are silent -- unlike on every third down the Jets defense forced Sunday at MetLife Stadium -- even with one of the league's best dual-threat quarterbacks, San Francisco's Colin Kaepernick, looming in six days.
And in keeping with the steady-as-she-goes mindset, Nelson doesn't plan to change much about his daily work. Given that his diligence helped him rise in the sub-package depth chart, it's clear Nelson is doing something right.
"Nothing changes. More, if anything, it's just a lot more focusing and cutting off more traction from here on out," Nelson said. "But nothing really changes. Everything -- the game plan, the way I work, the way I study throughout the week -- is still the same."
And as they face the prospect of another the Broncos hope the results are the same for a defense that ranks fourth in the league in yardage per game, third in yards allowed per play, fourth in rushing average per play and third in net passing yards per play.
The ascending defense is en route to a revival season after last year's struggles. It is up to Marshall and Nelson to ensure no deviations from the flight plan.