After BYU plays New Mexico State on Saturday (8:15 p.m., MT, ESPNU), it has a bye week before facing Hawaii on Dec. 3.
That's the earliest that Nelson could return to the field. But even if Nelson is cleared to play, he still might not be able to make the trip to Honolulu, Mendenhall said.
The coach also updated and clarified the nature of Nelson's injury and when it occurred.
Following BYU's 42-7 win over Idaho, Mendenhall said Nelson was hurt when he absorbed a hard hit while throwing a 32-yard touchdown pass to Cody Hoffman in the first quarter. On Monday, Mendenhall said the injury actually occurred four plays earlier, when Nelson was hit right after throwing a pass that was nearly intercepted.
Nelson continued playing, and on the touchdown pass, he was hit again as he released the ball. Nelson was checked by medical personnel, then taken to the hospital. Nelson suffered two broken ribs, Mendenhall said, and a partially collapsed lung. He stayed at the hospital over the weekend and was expected to be released on Monday.
"The severity of the blow hits you so hard that air escapes the lung and goes into the body cavity and then the lung collapses," Mendenhall explained. "About 30 percent of the lung was collapsed. (Doctors) put a tube in, sucked the air out, so the lung could re-expand. He'll be released from the hospital (Monday). From my understanding, there were two ribs broken or fractured near the top."
Mendenhall said "there's a chance" Nelson could play against Hawaii. However, even if Nelson is given the go-ahead to play by doctors, he might have to stay in Provo because the airplane cabin's air pressure on the long flight to the Islands could negatively affect Nelson's lungs.
"One of the issues will be, any time there's a lung injury with extended air travel, that adds increased risk," Mendenhall said.
With Nelson out, sophomore Jake Heaps will start his first game in nearly two months on Saturday.
"Jake has a great opportunity. It's just a matter of continuing to play with poise and maturity and increased confidence in leading our team," Mendenhall said. "Jake, much like Riley a year ago, had the more difficult part of the schedule to begin with. As now he has the chance to re-emerge … I would expect him to do it now after having watched at a level of increased leadership, excitement, execution and really demonstrate not only to his team but to everyone around BYU football to remind them how capable he really is. He showed that Saturday and I think he'll show that this upcoming Saturday."
Offensive lineman Marco Thorson said he and his teammates are going to rally around Heaps.
"We love Jake. Jake brings a unique talent to the team. When he's back there, he has an innate ability to make plays. He's a competitor. He'll pick us up. We'll play hard for him. (Heaps is) improved. He's more confident. He seemed calm back there (last Saturday)."
In relief of Nelson, Heaps completed 15-of-20 passes for 185 yards, with two touchdowns and an interception against Idaho.
The offense will undergo a transition of sorts in switching from Nelson, a running quarterback, to Heaps, who is a pocket passer.
"It is an adjustment. There will be have to be some things changed," Mendenhall said. "A lot of what Riley does, there are a number of running plays that make him effective. But really, the offense is similar enough right now that Riley's simple ability to scramble is what makes it so difficult (for opposing defenses)."
It's somewhat ironic that Nelson was injured while throwing a pass because of his aggressiveness in taking on tacklers head-on while running with the ball.
"It's not that he was just throwing it, but he was standing in until the very last second with a guy bearing in on him," Mendenhall said of the second hit on Nelson. "He sees him bearing in on him and stands and throws it anyway and throws a touchdown."
Mendenhall added that Nelson didn't want to come out of the game, despite dealing with intense pain. "His pain threshold and tolerance is so high," Mendenhall said. "He doesn't act hurt. When (medical personnel) pressed on his chest cavity on the sideline, they could tell that air had escaped the lung and that's when they took him to the hospital. It wasn't him saying he was in dire pain."
So, with Nelson sidelined, the offense, for now, is in Heaps' hands.
"I'd like (Heaps) to continue to improve," Mendenhall said. "After almost every drive on Saturday, I had just a quick thought or something to share with him. This isn't about his inability to play, this is simply about his ability to continue to learn and grow and be tutored and mentored over time, knowing that he is basically the equivalent of a redshirt freshman this year, with a lot of great football ahead of him. He played very well last year and will continue to play well I think this year based on when Riley can come back. Or maybe if (Nelson) doesn't, (Heaps) may be our quarterback that finishes the season."
Cougars on the air
New Mexico State (4-6)
at BYU (7-3)
Saturday, 8:15 p.m. MT
TV: ESPNU Radio: 1160 AM, 102.7 FM