"I pulled a muscle on my downswing trying to hit it hard," Mickelson said a statement. "It just killed and it wouldn't subside for 10 or 12 seconds. I'm going back to San Diego (for) a couple of days and have a doctor look at it, but there's really not much you can do for a pulled muscle. I hope I'll be OK to play the Shell in Houston, but I just don't know."
Mickelson is entered in the Houston Open next week, the last event before the Masters.
Lefty was 1-over par in the round and 4 over overall when the three-time Masters champion was taken off the course in a cart. His caddie, Jim Mackay, said Mickelson felt a twinge on his tee shot on No. 1 — his 10th hole — and that the pain wasn't going away.
"It's definitely not his back," Mackay said. "It's his right side. The thing he had been talking about is how good his back had been feeling here. He'll definitely play Houston, if he can."
It's the second time this year Mickelson has withdrawn in the middle of a tournament. He pulled out at Torrey Pines after making the cut because of a back injury.
On Friday, Mickelson made the cut on the number, following an opening 77 with a 70 at TPC San Antonio. He blasted out of a greenside water hazard to 5 feet to set up a birdie on the final hole.
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