Kusal Perera has retired hurt on day three after copping a nasty blow to the side of the helmet from Australian quick Jhye Richardson.

Perera was struck after turning his head to a Richardson bouncer that clattered into the batsman's helmet and sent the stem guard of pieces of the protective equipment flying.

He was immediately checked by the Sri Lanka medical staff and batted on with a new helmet.

But after facing up to four more balls from Richardson and another pitch-side check from the tourists' medicos between overs, Perera left the field from the non-striker's end three balls into the following over.

He departed on 29 with the score at 3-157 in the 54th over.

A Sri Lanka Cricket spokesperson confirmed Perera will undergo a mandatory concussion test as a precaution.

Replacing Perera was Dimuth Karunaratne, the opener who retired hurt on day one after being struck on the back of the neck by a Pat Cummins delivery.

Karunaratne was stretchered from the field and rushed to hospital. He was discharged late on Saturday night and was cleared of concussion.

He resumed his innings on 46.

Australia No.6 Kurtis Patterson said his team was in shock when Karunarate was felled on day two.

"You never like seeing that," Patterson said yesterday.

"You want your fast bowlers to obviously be aggressive and try and set them up to get the nick.

"But you never like it when players go down like that.

"All of us were in a bit of shock but I think everyone is OK, which is good."