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 defenceless. It's a tough area. It wasn't a blatant (example of) run a guy from behind."