"He's not scared," Coyotes coach Rick Tocchet said.
He wasn't then. He isn't now.
He had played three games last season, jumping into the lineup immediately upon signing a contract with the Coyotes after finishing the season at Boston University, had two assists in those three games, had given the team a glimpse of what was to come.
"I worked all summer toward that one goal, to make the team [this season], and when I was at camp I knew that I was ready and that I should be on this team and I will be on this team," said Keller, who was selected No. 7 by the Coyotes in the 2016 NHL Draft. "So it was never a doubt in my mind that I wasn't going to be on this team."
No one, however, could have predicted how Keller would start this season.
"I mean, you're just so happy about it and you're so proud of him, but every once in a while you just go, 'gosh, is this real?' " Kelley said.
It is.
This, after all, is a player that many in hockey compare to Chicago Blackhawks forward Patrick Kane without a sense that it might be putting too much pressure on a kid who is not much more than that, a kid.
"He's just such a dynamic player," former BU teammate and current Boston Bruins defenseman Charlie McAvoy said. "He has Patrick Kane-like skill with hands and vision and his shot, he's got a deceptive release, he's got deceptive speed. Almost just like Patrick Kane.
"You think of Clayton and his potential and what he could be like, he could be a next-generation Patrick Kane."
He has all of that, the skills, the hockey sense, the smarts, the vision, the shot, and he does it all effortlessly. He can battle in the corners with a ferocity belying his size, the 5-foot-10, 170-pound frame that invites the Kane comparisons.
"He's got the two things that you need to be special: He's got elite talent and he competes at an elite level," BU coach David Quinn said. "His motor never stops and he's tough, he's hockey tough. When you combine his hockey IQ and his skill, which is elite, with his elite competitiveness and how tough he is, you have a special player, and I don't care how big he is."
Especially because in today's NHL, as Quinn pointed out, players don't have to be big anymore. They need to be quick and skilled and fierce. They have to hound the puck. And that is Keller, a player good enough that he's second in the NHL in goals by a rookie this season, tied at 12 with Alex DeBrincat of the Blackhawks and five behind the Vancouver Canucks forward Brock Boeser.