Representative Ronny Jackson, Republican of Texas, was on Donald J. Trump’s plane on Sunday afternoon, the day after the former president survived an assassination attempt, when he quickly reverted to his role of doctor.

Mr. Jackson, who was Mr. Trump’s White House doctor during his presidency and parlayed their friendship into a successful run for Congress, replaced the dressing on the former president’s ear on a flight to Milwaukee for the Republican National Convention.

“The bullet took a little bit off the top of his ear in an area that, just by nature, bleeds like crazy,” Mr. Jackson said in an interview. “The dressing’s bulked up a bit because you need a bit of absorbent. You don’t want to be walking around with bloody gauze on his ear.”

Mr. Trump entered the convention arena in Milwaukee on Monday night wearing a bulky white dressing on his right ear. So far, only Mr. Trump has described his injuries; his team has not provided any formal medical briefing to the public since the shooting.

Mr. Jackson described in the interview the surreal moment when he learned a gunman had tried to assassinate his friend and former patient during Mr. Trump’s outdoor rally in Butler, Pa., on Saturday.

Mr. Jackson was in his bedroom in Amarillo, Texas, on Saturday night, packing his bag for the Republican convention. He was watching the Trump rally on his iPad, but he had stopped the livestream a couple of times, so he lagged behind Mr. Trump’s remarks by a couple of minutes. His wife, Jane, was watching the rally on a television in the living room, and she was speaking to someone on the phone. She suddenly called out to him.