Graeme McDowell's involvement in this year's Masters has been thrown into doubt.

The Ulsterman has been forced to withdraw from the opening round of the Valero Texas Open, after aggravating a stress injury to his lower left leg.

He was on 5-over par after nine holes.

Strong winds are playing havoc with the field in San Antonio where Shane Lowry will tee-off later this hour.

Just two players are below par at the moment.