The former BMW PGA champion withdrew just before the start of the tournament and plans a scan to see the extent of the damage to his wrist.
"I went on the waterslide with my daughter (seven-year-old Ruby) in Portugal and didn't think it was scary until I was about halfway down," said 40-year-old Khan.
"I landed on my wrist, but I don't know how much it contributed - I've had a bit of a problem before.
"I played the pro-am, but the last few holes I couldn't hit it properly and after having treatment I hit about 10 wedges and that was enough."
His place went to Australian Daniel Gaunt, who made it onto the leaderboard at three under par after nine holes.