Moore is out indefinitely.
Moore left the Broncos' 27-17 win over the Kansas City Chiefs in the first half Sunday night and was replaced by veteran Mike Adams.
During the night, Moore called the team's athletic trainer, Steve Antonopulos, because he was still experiencing pain and swelling, interim coach Jack Del Rio said. Antonopulos arranged for Moore to be examined by the team's medical staff, which determined he needed what's called a fasciotomy.
That's where a surgeon cuts open the skin and fascia covering the affected compartment, which are groupings of muscles, nerves and blood vessels.
Compartment syndrome develops when swelling or bleeding occurs within a compartment, which the American Academy of Orthopaedic Surgeons says can be caused by athletic exertion or an impact injury.
It's rarely seen in football players, however.
"It's a new one for me, I'm learning about it," Del Rio said. "I had to write it down so I can say it properly. It's just one of those freakish things that can occur — and it did.
"And fortunately he called Greek (Antonopulos). I think Greek's as good as anybody out there, and they got the doctors right on it and he went in and had the surgery. It can be a very serious injury, it could lead to a loss of a limb, and potentially a loss of life, so it's a very significant injury."
The Broncos couldn't pinpoint when Moore first began having problems with his lower left leg.
"It could have been something that started during the week," Del Rio said. "It could have been something that started during pregame warm-ups. I don't know. I'm not sure anyone knows. But at some point he ended up with basically a muscle that was bleeding."
Moore, who remained hospitalized Monday, will not play Sunday when the Broncos (9-1) visit the New England Patriots. Del Rio said he had no timetable for Moore's return.
"The expectation is that he'll be back at full strength at some point, I just don't know what that timeframe is," Del Rio said.
Cornerback Chris Harris said one play in particular let him know that Moore was injured: Jamaal Charles' 35-yard run around the left side in the second quarter.
"I knew something had to be wrong with him when Charles busted through on that long run and he didn't have that acceleration to go make the tackle," Harris said. "That's when I realized that something was bothering him."
Moore left the game after that and didn't return.
Moore has had a resurgent season after being the goat in Denver's double-overtime loss to Baltimore in the playoffs in January. He allowed Jacoby Jones' 70-yard touchdown catch in the final minute of regulation in that game when he went for the interception instead of the tackle and mistimed his leap at the 20-yard line.
This year, he's been much more disciplined in his coverage and has honed his open-field tackling techniques, collecting 44 tackles and two interceptions.
"He's been strong, mentally," Del Rio said. "I think he's kind of put his head down and gone to work. He was off having a real good year, a solid year. He's had some interceptions and he's making a lot of tackles. The one thing he's really worked hard on is taking good angles on tackles out of the middle of the field and minimizing big plays. I think that's been a real positive for us."
Del Rio also said tight end Julius Thomas (knee) was day to day but had no update on slot receiver Wes Welker (concussion), who needed to pass the league-mandated concussion protocol Monday in order to be able to practice by Thursday and have any chance of facing his former team this weekend.
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