A 44-year-old grand-father five years removed from his last NFL game, Philip Rivers, stood at a podium in Indianapolis on Wednesday to explain why he decided to play professional football again after retiring in 2020.

“I know that there is risk involved in what may or may not happen,” Mr. Rivers told reporters in his familiar drawl. “But the only way to find out is to go for it. I just felt like it was a gift, another opportunity to play and cut it loose with the guys in a game you love to play. It’s a dream I got to live, and now you may get some bonus time.”

The former Chargers and Colts quarterback was signed to Indianapolis’s practice squad on Monday after a rash of injuries to the team’s signal-callers. Starter Daniel Jones, who was enjoying a season of redemption, injured his Achilles tendon against the Jacksonville Jaguars on Sunday and is out for the season. Backup Riley Leonard, a rookie, is nursing a knee injury, while Anthony Richardson, a third quarterback, is already on injured reserve.

The Colts reached out to Mr. Rivers to gauge his interest in coming back. The father of 10 has been coaching high school football in Alabama since his retirement and wasn’t holding out any hope of playing again. “I thought that ship had sailed,” he said.

But along with coaching, he served as a mentor to Mr. Leonard and kept tabs on the Chargers, a team he played for from 2004 to 2019, and the Colts, for whom he played in 2020. When the Colts called, he said his competitive instincts kicked in.

He agreed to a tryout at the Colts facility on Monday, and despite admitting to being “a little scared and a little nervous,” he was signed to the practice squad and is preparing to play, if needed, in Sunday’s game against the Seahawks at Seattle.

“The decision wasn’t easy because of all the things you can imagine,” Mr. Rivers said. “It’s human nature to have a little doubt. That’s normal. Who wouldn’t have doubt after five years. But the only way to find out if you can do it, is to go try.”

It helps that he knows the Colts’ offense and is comfortable returning to a familiar building and rejoining former teammates. After passing for 63,440 yards and 421 touchdowns during his career, Mr. Rivers said he can still throw a football. It’s the rest of the stuff that might take time to work out the kinks, he said.

“It’s one of those deals where a door opens and you can either walk through it and find out if you can do it or run from it,” he said, adding, “I know five years is a long time. That weighed on me a lot with it being so long.”

The Colts are in a tight race for a playoff berth. The Jaguars lead the AFC South at 9-4 with the Colts and Houston Texans at 8-5. Indianapolis’ next four games against the Seahawks, San Francisco 49ers, Jaguars and Texans will decide the team’s fate.

If Mr. Leonard is healthy enough to play, he’ll start. Should Mr. Rivers take the field he’ll become the second grandfather to play in the NFL, following Brett Favre, who became the first in 2020.

If activated, Mr. Rivers would become the oldest player in the league, a spot currently held by Aaron Rodgers of the Pittsburgh Steelers, who is 42.  Tom Brady was 45 at the time of his retirement, and George Blanda played until he was 48.

Mr. Rivers admitted his wife Tiffany is concerned about the risk of injury.  “She’s nervous about the physical aspect as you would expect any wife to be,” Mr. Rivers said. “But whether you’re 24 or 44 anything can happen. That’s never been a concern for me. If something like that happens, I’ve got a long time to recover.”

His children are excited. Some were too young to remember his playing career. Others were teenagers. He doesn’t expect to be the player that was voted to the Pro Bowl eight times. But he might be good enough for now.

“I am not naïve to the challenge ahead,” he said.  “As long as I can live with the results both good or bad and go back home and move on, I’m willing to go for it.”