And it showed.
The senior quarterback completed only 4-of-9 passes for 19 yards, with three interceptions and one fumble, and was replaced by true freshman Taysom Hill in the third quarter.
"I watch the film and I know that I'm not playing my best," Nelson said Monday. "Compare my film how I played against Washington State to how I played against Boise State. It doesn't look like the same player. I recognize that. However, if the coaches decide that I give us the best chance to win, then I want to be out there."
Nelson, who is dealing with a significant back injury suffered on Sept. 8 against Weber State, participated very little in Monday's practice.
But Cougar coaches say the plan is for Nelson to start Friday (6 p.m. MT, ESPN) against Hawaii — if he's healthy.
"Riley's a little bit beat up still," said coach Bronco Mendenhall. "It might take until the end of the week before we know for sure if he can play or not. But nothing's changed in my mind (about who the starting quarterback is)."
"Each day there will be an evaluation on him," offensive coordinator Brandon Doman said of Nelson. "We'll rely heavily on him, and our doctors and trainers to see how he develops. If he's not healthy and ready to go this week, I doubt we'll play him. If he is, we'll play him. I think that will all be dependent upon how he does the next 36 hours."
Asked Monday to put a percentage on his health status, Nelson said, "Boy, not very high. I really want to play and really want to be out there and battle with my guys. Today? It's better than it was Thursday. Thursday I'd say 65 percent. Today, I'd say 75-80."
If Nelson is unable to play, Hill, who led the Cougars' only scoring drive against Boise State, will make his first career start.
"I have no question that Taysom will go in and play well and do just fine if he plays," Doman said.
Hill took the majority of reps in Monday's practice. "If the call does come to put me in," Hill said, "I'll be ready and prepared."
In the quarter-and-a-half he played against Boise State, Hill completed 4-of-10 passes for 42 yards, and he rushed for 72 yards and a touchdown.
"I made a couple of bad throws and moved on," Hill said. "As far as everything else, I felt like I managed the game pretty well."
Meanwhile, Nelson is hoping to return to the field.
"I'm going to do all I can to work with our medical staff and manage reps in practice and everything with coach Doman and coach Mendenhall to be able to do so," he said. "It's my senior year and my opportunities are very few. I hope to take advantage of every one."
And because of his fierce competitiveness, Nelson said he would never admit that he isn't able to play. "I don't think I could ever bow out," he said. "I don't think I could ever throw in the towel."
Nelson acknowledged that the pain in his back affected his ability to make plays against Boise State.
"It affects you physically but also mentally. There were a few throws that psychologically I didn't have as much confidence in because I could feel the injury," he said. "I would be lying if I said there was no effect. However, that is no excuse. If the coaches are going to put trust in me to put me on the field, I need to be able to perform at the highest level. I wasn't able to do that … During the game, you just manage (the pain) the best you can. Being honest, it does take up a lot of your energy and a lot of your focus is dedicated to pain tolerance when you're playing with an injury."
"He's tougher than any of us," Mendenhall said of Nelson. "Medical personnel work on him every day all week to get him to where he can play. It's hard for him to make it through. But I believe in him and I'd like him to be our quarterback. If he's close to being able to go, we'll play him. If he can't go, then he won't play. It's similar to the past couple of weeks … He'll go until he can't hardly stand up. We have to know that and help him make that decision."
Doman said Nelson's back is healing and getting better.
"It's significantly better right now than it was a week ago. I would suspect that by (Friday) he'll be pretty close to back to normal. That will be a tough decision to make. Do you prolong it? Or do you sit him down and let it get better? We'll have to make that decision soon … Hopefully we'll make the right decision."