A Los Angeles Chargers team doctor accidentally punctured quarterback Tyrod Taylor's lung just before kickoff Sunday while trying to administer a pain-killing injection to the quarterback's cracked ribs, league and team sources told ESPN.

That is the reason Taylor did not start Sunday and rookie quarterback Justin Herbert unexpectedly did against the defending champion Kansas City Chiefs, in an arrangement that is likely to continue Sunday against the Carolina Panthers and quite possibly beyond.

It also helps explain why Chargers coach Anthony Lynn strongly backed Taylor on Sunday night and again this week. Many coaches believe that no player should lose his job to an injury -- especially one that accidentally came at the hands of a team doctor. The Chargers previously said Taylor suffered complications from an injection. Those complications were a punctured lung.

The NFL Players Association is investigating the medical mishap, according to NFLPA assistant executive director of external affairs George Atallah.

"Our medical and legal team have been in touch with Tyrod and his agent since Sunday collecting facts," Atallah tweeted Wednesday. "An investigation has been initiated."

The injection that Taylor received is not entirely uncommon, but it carries a risk because a doctor injects without seeing where the needle is going. Still, it is rare that a player suffers a punctured lung from the injection, especially right before kickoff, sources said.

Taylor suffered two cracked ribs early during the Chargers' Week 1 win over the Cincinnati Bengals, but he didn't have an MRI on the injury until later that week. Taylor was added to the Chargers' injury report with a rib injury Friday, but after he was a full participant in practice that day, the quarterback was removed from the report.

He planned to take a pain-killing injection for the rib injury Sunday, but after the Chargers doctor punctured his lung, Taylor wound up in the hospital Sunday night to be treated for the injury and the struggles he had breathing.

Doctors have advised Taylor not to play "indefinitely." Lynn said Monday that "if Tyrod is 100%, he's our quarterback."

The Chargers want Taylor to continue as their quarterback because of his production and reliability. He has a winning record as a starting quarterback, didn't turn over the football in Week 1 and has the trust and respect of his teammates.