England's Jos Buttler said he "feared the worst" when his shot struck Sri Lankan fielder Pathum Nissanka on the head, resulting in hospital treatment.
The incident occurred on the final day of England's drawn match against a Sri Lanka Board President's XI.
"You're just worried, and that is the nature of it," said the 28-year-old, who was given out when the ball looped off the helmet and to Angelo Mathews.
"Short leg is a thankless position to field. Let's hope he's not badly hurt."
Nissanka, 20, received treatment from England's medical staff for about 15 minutes before he was taken off on a stretcher, with the players taking an early tea.
England Cricket tweeted that the Sri Lankan was conscious as he left the field and had gone to hospital.
Buttler said: "You always fear the worst, I think.
"I hit him very hard and straight in the middle of the helmet and luckily the medics rushed straight on and treated him.
"You don't mean to cause injury - it is a really unfortunate accident."
England drew the two-day match in Colombo. They have another warm-up match, which starts on Thursday, before the first of three Tests gets under way in Galle on 6 November.