Horschel reportedly "tweaked" his back in Florida before departing for Shanghai, and after stretching to prepare for a Tuesday practice round he opted to withdraw from the no-cut event where last place receives $40,000. Per a PGA Tour official cited in the report, the injury is not considered serious.
Horschel hasn't played competitively since a T-32 finish at the BMW Championship. After winning the FedEx Cup in 2014, his most recent season included only two top-10 finishes in 28 starts, including a third-place finish at the Valero Texas Open in March.
Horschel will be replaced in the 78-man field by 22-year-old Cameron Smith, who gained notoriety with his T-4 finish at the U.S. Open. England's Eddie Pepperell is now first alternate.