Pominville sustained a concussion and a gash above his eye that required 7-8 stitches. He was diagnosed in the building without going to a hospital.
With the Hawks trailing 2-1 and 5:42 remaining in the first period, Hjalmarsson hit Pominville from behind just inside the Sabres' zone, sending him flying into the boards where his head hit the glass violently.
"Everything happened so fast, but I thought it was shoulder to shoulder," Hjalmarsson said after the 4-3 win by the Hawks on Monday. "My intention wasn't to hurt him and I heard our doctor said he was up walking so that's a big relief."
Hjalmarsson was given a five-minute-major penalty and a game misconduct, forcing the Hawks to play with five defenseman for the rest of the game.
Pominville didn't move for several minutes, laying on his back on the ice as emergency medical personnel and Sabres trainers slipped a flat board underneath him to get him on the stretcher, and his head and neck were stabilized. He gave the fans a "thumbs-up" sign on his way off the ice.
"It was a terrible hit," Sabres left wing Thomas Vanek said. "It was to the head and from behind, but [Hjalmarsson's] not a dirty player."
Hjalmarsson could face further punishment by the league, but Blackhawks coach Joel Quenneville is hopeful there won't be additional penalties.
"You could argue if it was even a penalty," Quenneville said. "He wasn't looking at the guy, [he] was playing the puck. He was compromised. But the hit, there was no intent at all."
Buffalo coach Lindy Ruff said he knows Hjalmarsson has a clean past, but also didn't let him off the hook.
"He's not that type of player, but he made a mistake, obviously," Ruff said. "He made a big mistake. He caught a player in a tough spot. It'll be addressed."
Pominville entered the game as the team's active leader in consecutive games played with 335.
Jesse Rogers covers the Blackhawks for ESPNChicago.com and ESPN 1000. The Associated Press contributed to this report.