Two more top-25 players in the Official World Golf Ranking withdrew Sunday from this week's PGA Championship.

Paul Casey of England and Sungjae Im of South Korea backed out of the field of the year's second major, with Casey still battling a back injury and Im testing positive for COVID-19.

Russell Knox of Scotland and Scott Stallings entered the field as replacements for Casey and Im, respectively. Joel Dahmen is now the first alternate should any other player withdraw before Thursday's first round at Southern Hills Country Club in Tulsa, Okla.

Casey has not been able to compete since placing third at The Players Championship in March. He showed up to the WGC-Match Play fighting back spasms and forfeited all of his scheduled matches in the initial group stage of the event. He was a late withdrawal from the Masters the next month.

Im reportedly tested positive for COVID Thursday before he was set to play in a tournament on the Korean Tour. His withdrawal indicates that he would not be able to travel back to the U.S. in time for the tournament due to South Korea's COVID protocol.

Im is ranked No. 20 in the world. Casey is No. 24.

World No. 25 Harris English withdrew from the field last Friday. English has not played on the PGA Tour since mid-January and is still recovering from a hip surgery he underwent in February.

Six-time major winner Phil Mickelson also dropped out of the field last week. Mickelson, who would have been the defending champion, has not returned to the public eye since receiving backlash for comments made about a nascent rival golf league backed by Saudi financiers.

--Field Level Media