"It didn't feel like work. I just wanted to get out of the house," he said. "The drugs kill your red blood cells in your body, and still they're not all back, and I remember in the gym older people flying past me. I was just like, 'Wow, I've got a long way to go.'"
And through it all, his teammates remained in touch, supporting him every step of the way.
"I really appreciated that throughout the whole thing," he said. "It shows how much character everyone has here."
"There are really no words to describe that, right? It's a fight that relates to everyone," said Haydn Fleury, who roomed with Mattheos during the Calder Cup Finals. "He wasn't alone in the process, and he's a young kid who has a bright future ahead of him."
On Friday, Sept. 20, a week into Carolina Hurricanes' training camp, the north-side doors of the locker room opened, and Mattheos walked in, his hockey bag and sticks in tow.
His Checkers teammates were the first to spot him. Fleury embraced him. So, too, did Julien Gauthier.
"Really cool. Really happy for him," Fleury recalled of the moment. "He fought all summer. It was good to see him in training camp."
The next morning, Mattheos laced up his skates and got on the PNC Arena ice.
"To get clearance and get back to doing what I love is an amazing feeling. It's great to see all my friends and my teammates here," he smiled. "I'm just excited to put this behind me and play hockey again."
Mattheos completed his comeback to hockey in early November, returning to the Checkers' lineup for a weekend set in Cleveland.
His story, though, is still only beginning.
Mattheos, selected by the Canes in the third round of the 2017 NHL Draft, aspires to one day play in the National Hockey League, of course. And as Hockey Fights Cancer, so too will Mattheos continue his fight, having already conquered one of life's biggest hurdles this past summer.
"I just want to be known as a hockey player, a good guy who plays hockey. It's a huge part of my life. It was scary for a little while because I didn't know what exactly the future was going to hold, but now I'm excited to be back," he said. "I'm not going to doubt myself. I've made it this far. I know it's going to be tough, but I'm not going to give up."