The defenceman had the primary assist on the Ottawa Senators' two goals as they clinched a playoff berth by beating the Washington Capitals 2-1 in overtime Thursday.
"Erik shows the difference that he can make and the quality and type of player that he is," Senators coach Paul MacLean said. "Obviously our team was a different team with him on the ice and the things that he can do that others can't do. He's a very special player, and to his credit he was up and running."
Karlsson played 27:11 as MacLean hardly eased the reigning Norris Trophy winner back in.
"Coach has a pretty good idea who to put out there and when to, and it felt good and my conditioning felt well and I think he recognized that and put me out there when it was my time," Karlsson said.
Karlsson said he didn't feel any pain during the game. That's progress after saying earlier in the day that he wasn't rushing back into the lineup.
The 22-year-old did not look like a player who had missed 31 games with the Achilles injury that doctors estimated would keep him out four-to-six months.
"We saw a little bit of what he can bring tonight," Capitals coach Adam Oates said.
Karlsson did not give rave reviews about his two-assist performance.
"I felt OK," he said. "Did battle some issues out there and didn't feel quite as comfortable as I used to. But overall I think it was a solid game. I still have to work through some mistakes and clean those up."
Karlsson said he struggled getting back up to speed with players who have been in a rhythm all season. But he took much more joy in the Senators clinching a playoff spot than playing in his first game since Feb. 13.
"I'm going to have to work on my stuff and as long as the team keeps winning I'm going to be happy," he said.
With a playoff spot wrapped up, Karlsson has two more games to fine-tune himself as Ottawa plays for seeding. But the first impression back in the lineup wasn't a bad one, and that's why MacLean didn't hesitate to put him on the ice.
"He was good and he was going," MacLean said. "The best players play, and I thought he was one of the best players."