INDIANAPOLIS -- With the Los Angeles Rams already enjoying a 20-0 lead on Sunday and the game quickly spiraling out of control, the Indianapolis Colts could ill afford an unforced error from their quarterback.

Yet, with 4:39 remaining in the second quarter, Anthony Richardson did exactly that, fumbling an accurate shotgun snap from center Wesley French. The ball inexplicably slipped through Richardson’s hands and popped into the air. By the time he corralled it, two defenders were converging and Richardson had no alternative but to give himself up after a 2-yard loss.

It wasn’t exactly the inspiring play the Colts needed at the time, but it all proved an opportunity to demonstrate the rookie quarterback's unflappable nature.

On the next play, Richardson dropped back and uncorked perhaps his best pass of the game: a deep throw down the seam to receiver Josh Downs perfectly placed over trailing defender Cobie Durant for a 30-yard gain. It was the Colts’ longest play of a first half marked by consistent struggles.

And it was only the beginning for Richardson.

The Colts would go on to lose against the Rams in overtime, but Richardson’s eye-opening performance in the second half showed he possesses rare composure for a rookie quarterback even in unideal circumstances.

“Once he gets rolling,” coach Shane Steichen said, “he gets rolling.”

In spite of the three-touchdown deficit, the two missing offensive-line starters and his underwhelming play early in the game, Richardson put together an impressive, game-tying effort in the second half, throwing for two touchdowns and running for a third to rally his team.

What the Colts are witnessing are the requisite ups and downs of a rookie quarterback. Where Richardson appears to be setting himself apart is that his crescendos are not isolated flashes, but something more sustained. On Sunday, it was sustained enough for the Colts to score 23 unanswered points.

“We shot ourselves in the foot multiple times,” said tight end Drew Ogletree, who caught one of Richardson’s touchdown passes. “After halftime, you could really look into his eyes and see he’s got that fire in him. He was ready to go. You can tell he’s going to be something special in this league for sure.”

Some of what Richardson has accomplished so far was somewhat predictable. He became the first quarterback in league history to record rushing touchdowns in his first three games (Richardson missed Week 3 with a concussion). He also became the first Colts quarterback to record a rushing touchdown in three consecutive games and the 21-year-old is the youngest quarterback in the Super Bowl era to record multiple passing touchdowns and a rushing touchdown in a single game.

Just as important, though, is the mettle he showed. Multiple Colts staffers were still in awe Monday morning. For every late decision or missed opportunity Richardson displayed early in the game, it seemed he made an equally remarkable play later on.

Among them: A 35-yard touchdown pass -- thrown on the run, off one leg -- to tight end Mo Alie-Cox in the third quarter; a Houdini-like escape and rollout throw on the ensuing 2-point conversion that led to a successful try; and a 22-yard, fourth-down completion with three minutes remaining while rolling to his left.

The Rams fooled Richardson with some of their coverages in the first half, showing some defensive looks the Colts did not anticipate. That’s typical of rookies. But Richardson quickly adapted and, later, thrived -- a promising sign.

“We were definitely frustrated in the first half,” Richardson said. “The offense, we started off very slow. We weren’t executing the right way or doing the right things that we thought we were going to do. They showed us a little bit of different things. It took us a while to adjust.”

More than anything, Richardson’s ability to prevent his early struggles from snowballing bodes well for the Colts. He was contending with a Colts defense that seemed incapable of stopping the Rams’ passing game (Los Angeles didn’t punt until the fourth quarter) not to mention the game-wrecking presence of All-Pro defensive lineman Aaron Donald, who exploited the weak spots in the Colts’ offensive line.

“[In] the second half, we just came out and tried to play and focus on the next play instead of worrying about trying to dig ourselves out of the hole,” Richardson said.

Richardson could have said the same about himself given the way he responded late in the game. It’s a good thing, too, because rookie quarterbacks make rookie mistakes, especially when down three scores. The key is what happens next.

The poise Richardson displayed on Sunday just might have provided the answer.

"The resilience he showed," Steichen said, "... was awesome to see from a young player."