Kaleta, who has had a history of head and neck problems, was boarded by Toronto's Mike Brown, who was assessed a penalty for the play. Kaleta skated off awkwardly with 2:26 left in the period and did not return.
"It's a neck injury," Buffalo coach Lindy Ruff said after the 4-3 overtime loss. "He was X-rayed here and they took a precaution. They took him for some further tests. I have no update."
The NHL has made hits from behind a point of emphasis in recent years, but Ruff didn't feel Brown's hit was dirty.
"I thought he was sideways," Ruff said. "Some of the obligation is on the player [being hit]. He wasn't defenseless. It's a tough area. It wasn't a blatant [example of] run a guy from behind."