Kovalchuk missed his team's 4-1 victory in Game 2 with what the team is calling a lower-body injury. The Star-Ledger of Newark reported that the 29-year-old is dealing with a herniated disk in his back.
Kovalchuk made an immediate impact Thursday, scoring and adding an assist in the first period.
Kovalchuk said he was injured in the Devils' opening-round series against the Florida Panthers, but declined to confirm the report about the back injury.
Hybrid winger/defenseman Peter Harrold was scratched to make room for Kovalchuk.
After resting for four days, Kovalchuk participated in the team's morning skate Thursday, and said he felt "very good."
"Why would you sit and rest if you feel better?" said Kovalchuk, who added that doctors and trainers "worked their magic" to get him back healthy.
DeBoer said the decision to play was basically up to Kovalchuk.
"For me, if he's healthy to go, he's healthy to go, it's not healthy with an asterisk," DeBoer said.
In eight playoff games this season, Kovalchuk has three goals and three assists.
The best-of-seven series is tied 1-1.