But if the rookie sensation was feeling ill, he certainly didn't show it on the field.
After a sluggish start to the game, in part thanks to an early Rapids press, Morris came through with a crucial goal in the 56th minute to pad the Sounders' aggregate lead, taking a pass from Nelson Valdez and using his right foot to chip it over the onrushing Zac MacMath.
"Great ball from Nelson. He got it 25-ish yards out, maybe," Morris told ESPN's Taylor Twellman immediately following the game. "Then I saw there was a little opening there and he slipped me in. And then I saw MacMath was coming off his line so I kind of tried to get it over him and fortunately it went in."
If the virus and 11 Rapids weren't enough for Morris to battle, he suffered an inadvertent cleat to the knee from MacMath on the goal that forced him to limp off the field for treatment, raising some concern from his father, who is the chief medical director and team surgeon for the Sounders.
"It’s so funny having him [my father] on the sideline. I was kind of getting mad at him there. I’m like 'I’m fine. Just put me back in,’" he added. "I would never yell at my dad off the field but on the field I was getting a little mad. But it’s fun having him around."
After all that, though, Morris was able to play out the full 90 minutes and found himself in jubilant spirits after helping secure a historic first MLS Cup appearance for the Sounders.
"I’m feeling OK," he said. "But obviously it’s so great that the team was able to accomplish this. And it’s for the fans. They deserve it."
Sunday's goal represented another chapter in a remarkably successful rookie season for Morris, who was a US national team player before the Sounders even inked him to a Homegrown contract in January. After going his first five games without a goal, he scored 12 times and added 4 assists while playing all 34 regular-season contests for Seattle, comfortably winning MLS Rookie of the Year honors.
Sunday's goal was Morris' second of the Audi 2016 MLS Cup Playoffs to go with one assist. His first playoff goal came in the first leg against Colorado.
"At the beginning of the year things weren’t going in and I was listening too much to the outside world," Morris said, reflecting on his rookie campaign. "And I put that all away. And once I got the first goal, I kind of got the monkey off my back. And I’m playing with more confidence. That’s the biggest thing."
He'll have to rely on that confidence – and the toughness he's shown throughout the postseason – one last time in 2016 when the Sounders face either Toronto or Montreal in MLS Cup on Dec. 10 (8 pm ET; FOX and UniMás; TSN and RDS in Canada).