But after 11 series in three games, Luck and his surgically repaired right shoulder handled being sacked and being hit while scrambling. He even threw a touchdown.
During a preseason when wins and losses don't mean a thing, the Colts and Luck can say they came out winners because there's no doubt that the QB will be ready for the team's Week 1 game against the Cincinnati Bengals on Sept. 9 like he said more than two months ago.
What grade did Luck give himself?
"I'm not going to give a letter grade. Sorry," Luck said, laughing, after the Colts' 23-17 win against the Niners. "Pass or fail? Pass."
There's still plenty of things for Luck to improve on before he plays in his first regular-season game since Week 17 of the 2016 season, but hearing him say he "passed" the preseason is exactly what everybody in the Colts organization needed to hear.
"Other than making sure he was good physically, I really didn't have very many questions," coach Frank Reich said. "I think you guys know how I felt about this thing from the very beginning. I just had a lot of confidence in him and the people around him. The process that he's going through, seeing him work every day, seeing his attitude, really had not too many doubts."
Luck, like many of the other key Colts players, won't play in the team's preseason finale against Cincinnati on Aug. 30.
He waited until his final series before leading the first unit to its lone touchdown of the preseason. It came on a perfectly thrown 15-yard pass to tight end Eric Ebron with less than nine minutes left in the first half.
What made the touchdown more impressive is that Luck threw it after taking a big hit from 49ers linebacker Dekoda Watson on a 15-yard scramble.
"To be honest, I forgot how fast guys in the league are," Luck said. "The guy from behind actually caught me by surprise. And it was a lesson again that in this league there's always someone behind you and you can't relax."
Being able to withstand a substantial hit, let alone being able to make a precise throw on the next snap, was another box Luck was able to check off. He also showed improvement knowing when to throw the ball away, when to slide and when to be aggressive. The Colts want him to avoid taking unnecessary hits.
Luck, who has been sacked 156 times in his career, admits he has to change his game a bit, but he is also going to be aggressive when he has to, like when he scrambled for 15 yards on third-and-13 to keep a drive alive. He was sacked four times in the preseason.
"His decision-making in that regard has been perfect," Reich said. "There's not one time he tried to get more than he should have and gone down, and appropriately went for it at the times that he could. I think he's handled that. Whatever determination he made in his mind about how he's going to adjust his play, I think he should feel good about where he's at."
Luck, who didn't have starting left tackle Anthony Castonzo (hamstring) in the three games, finished the preseason 20-of-32 for 204 yards and a touchdown pass.
"I feel great, my shoulder feels awesome, the best it's felt," Luck said. "I actually feel very good, strong, my shoulder felt alive. I was a little happier with where the ball went when I threw it. I keep feeling and seeing improvement. Keep working at it."