GREEN BAY, Wis. -- Aaron Rodgers stared up at the Levi’s Stadium scoreboard, a wide smile stretching across his face as he watched the replay.

The Green Bay Packers star quarterback may not be able to decide on a cinematic nickname for undrafted rookie quarterback Joe Callahan -- “Bo” (for the fictional Wisconsin quarterback in “Draft Day”) or “Big Tom Callahan’s Son” (for Chris Farley’s classic character in “Tommy Boy”) -- but Rodgers seems sure of this: The kid can play.

“It’s fun to see him progress,” Rodgers told reporters following the Packers’ 21-10 preseason victory over the San Francisco 49ers Friday night in Santa Clara, California. “He’s constantly making improvements with his fundamentals, and he’s a gamer. He goes out there and makes plays.”

He certainly did against the 49ers.

Taking over for Rodgers three series into the game with No. 2 quarterback Brett Hundley still sidelined with an ankle injury, Callahan completed 16-of-24 passes for 167 yards and one touchdown (100.5 passer rating). And while the play that left Rodgers grinning skyward had been Callahan’s 18-yard touchdown pass to Jared Abbrederis midway through the third quarter, that wasn’t even the best play Callahan made during his seven-series audition.

On the opening drive of the fourth quarter, the Packers faced third-and-20 from their own 30-yard line. Against a blitz, Callahan miraculously eluded 49ers rushers L.J. McCray and Corey Lemonier, rolled to his right toward the Packers sideline and unleashed a 51-yard rainbow that Geronimo Allison caught at the San Francisco 19.

“We knew we had to get 20 yards to get the first and it was going to take a little time for our receivers to get downfield," Callahan explained. "They brought pressure inside. I was able to escape out, and I extended the play a little bit. I saw G-Mo down there and gave him a chance with the ball, and he made a great catch.”

The next play, undrafted rookie running back Brandon Burks scored the Packers’ final touchdown of the night.

“I thought Joe made plays. The [51-yard] throw [to Allison] was the play of the game, in my opinion, from our sideline,” Packers coach Mike McCarthy said. “He’s doing a better job of managing the huddle, commanding at the line of scrimmage and so forth. But there’s a lot of moving parts going there. What’s nice is he’s taken advantage of his opportunities. He’s improved each week.”

Perhaps even more exciting than Callahan’s throw to Allison was another across-his-body strike he threw earlier -- on an incompletion. On first-and-10 at the 49ers’ 38-yard line, Callahan rolled left after a play-action fake and threw long to Davante Adams, who was open on a post route but couldn’t make a leaping catch at the goal line. While the degree of difficulty was high, it was a catch Adams surely thought he should’ve made.

Rodgers apparently thought so.

“I told him, ‘Don’t let Davante ever forget that because that was one of the sweetest throws I’ve seen,’” Rodgers said with a smile.

While Callahan is a remarkable training-camp story, it’ll be interesting to see what the Packers do if Hundley’s gimpy left ankle isn’t 100 percent in time for the team’s Sept. 11 regular-season opener at Jacksonville.

Hundley has 15 days to heal before then, having originally injured his ankle during an Aug. 1 practice and re-injured it in the first quarter of the Packers’ Aug. 18 game against Oakland. Although Hundley had no discernible limp walking around the locker room last week and was standing on the sideline charting plays on Friday night, the Packers could be forced to carry Callahan on the 53-man roster if they have concerns about Hundley’s readiness.

Or, if they don’t think he could get the job done in a game that counted -- to be fair, Callahan also had his misfires, including a poor throw and questionable decision on a fourth-and-goal throw into the flat on the final play of the first half -- they’d have to scour the waiver wire for a veteran quarterback released on the final cutdown. The only other quarterback in camp is another undrafted rookie, Marquise Williams.

“Those are really decisions for the future,” McCarthy replied when asked if Callahan could be the No. 2 if Hundley isn’t ready for Week 1. “We’re not picking the team tonight. I think you can say that for all of our positions.”

Nevertheless, while Callahan is trying to make the unlikely leap to the NFL from tiny Division III Wesley College, he continues to show that the game isn’t too big for him, even if he still has plenty of room for improvement. Ideally, one would think the Packers would like to get him onto their practice squad and keep developing him, especially since they have so much depth at other positions that they’ll almost certainly want to steal an extra active roster spot by carrying only Rodgers and Hundley on the 53.

Through three preseason games, Callahan has completed 64.1 percent of his passes for 356 yards with two touchdowns and no interceptions (89.1 rating).

“I look at [Callahan] just like the rest of these young players; they have so much more to learn, and your time is kind of coming to an end with [some of] these guys,” McCarthy said. “But the improvement that they’ve made as a group, it’s been exciting.”