Lilja, who turns 32 in October, has quite a bit of familiarity with Manning after playing guard for six seasons in Indianapolis.
And while Jeff Saturday was Manning's customary center with the Colts, he's now working as an ESPN analyst.
The 6-foot-2, 290-pound Lilja made a similar switch last season as he went from guard to center for Kansas City. After the season, he announced his intention to retire.
But his retirement didn't last all that long.
Koppen tore his left ACL in a drill last weekend after the Broncos brought him back last month to fill in for J.D. Walton, who's had a setback in his return from ankle surgery.
Manny Ramirez, who took most of the snaps at center during Denver's offseason practices, was working with Manning the last few days in training camp. He sent a few hikes high, but nothing out of Manning's reach.
The team also recently brought in Steve Vallos, a six-year veteran who has played in Seattle, Cleveland, Philadelphia and Jacksonville.
"We always hate losing people for the season, but we will march on," coach John Fox said.
These days, the offensive line responsible for keeping Manning safe and secure is pretty banged up. All-Pro left tackle Ryan Clady has been limited at training camp due to a right shoulder. Tackle Orlando Franklin (toe, shoulder) and guard Chris Kuper (ankle) are also coming off surgeries. Franklin is back with the starters, while Kuper is going through drills on an adjacent field in shorts.
On Wednesday, the Broncos received another scare when cornerback Dominique Rodgers-Cromartie went down with a left ankle injury and had to be carted off the field.
"I am not over concerned," Fox said after practice. "But any time a player misses time, I am concerned. We will just get him healthy and get him back out there."
Guard Justin Boren also had to be helped off the field with a big toe ailment.