Why did Angelo Mathews retire hurt during 1st Test vs New Zealand?

Veteran Sri Lanka batter Angelo Mathews was forced to retire hurt during the first session of the first Test against New Zealand in Galle on September 18 (Wednesday). He was hit on his index finger by Blackcaps pacer William O'Rourke in the 22nd over of the innings. Mathews was batting on 12 off 55 when he retired hurt.