Nissanka took evasive action but could not avoid being struck on the helmet by Buttler's powerful pull shot and required treatment on the field before being carried off on a stretcher to a hospital in Colombo.
"You always fear the worst, I think. I hit it pretty hard and hit him flush so it was a big worry straight away," Buttler told reporters at the Nondescripts Cricket Club ground.
The incident on the second day of the two-day clash revived memories of the on-field injury suffered by Australian batsman Phillip Hughes in 2014 which proved fatal, but Sri Lanka Cricket (SLC) confirmed Nissanka was out of danger after an MRI scan.
However, the 20-year-old would remain in hospital for 24 hours on observation, SLC added.
"When he went down you are hoping for the best. The doctors and physios ran out and gave great care straight away. You don't mean to cause injury but it's a real unfortunate accident," Buttler added.
England next head to Galle, which will host the first match of the three-test series between the tourists and Sri Lanka from Nov. 6. (Reporting by Shrivathsa Sridhar in Bengaluru; Editing by John O'Brien)
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