Nominal Roll Update - William Sadler

While updating the nominal roll, I came across the curious circumstances of Driver William Sadler's broken tibia and the subsequent reporting of the injury.
Driver Sadler was a member of 38th Battery, 10th Brigade Canadian Field Artillery on 20 May 1917 when he was evacuated to hospital with a broken leg. The injury subsequently had him invalided to England for recovery. 
A Court of Inquiry was convened on 29 June 1917 during which witnessed described the break as having happened during an  organized football match between the 38th and 39th Battery on 20 May. Driver Sadler appeared to have stepped into a shell hole and broken his leg.
The attending Doctor wrote on the 28th May that the injury had been caused by a mule that had fallen on Driver Sadler's leg while he was delivering rations to the trenches.
I wonder which is the true story?