With veterans Raja Bell and Josh Howard already sidelined, Jazz decided to sign Ahearn, who averaged 23.8 points and 5.1 assists in 37 games with the Reno Bighorns this season.
Ahearn shot 40 percent from the 3-point line and established a league record by making 110 straight free throws.
"He's a great shooter," said Jazz coach Tyrone Corbin. "… We need perimeter shooting and he may be able to help."
According to general manager Kevin O'Connor, Ahearn has the ability to play point guard and shooting guard.
"… We saw him in the D-League Showcase [in January] and he showed improvement in all aspects of his game," O'Connor said.
The Jazz will be without Watson and Miles "indefinitely," said a team spokesman.
Watson suffered a torn meniscus in the second quarter of Utah's loss in San Antonio. Miles suffered a "moderate" left calf strain, also in the second quarter.
Miles was in a boot and walking with the aid of crutches before Monday night's rematch with San Antonio. He will be re-evaluated in a week.
Asked about the injuries to Watson and Miles, Corbin said, "They've been great for us all year. We're really going to miss them. … We support them fully and, hopefully, they recover quickly. But we'll miss them while they're out."
Spurs rest Big Three
The Spurs played the Jazz on Monday night without their Big Three of Tim Duncan, Tony Parker and Manu Ginobili. They were not injured. Instead, coach Gregg Popovich rested them.
"We're trying to survive," he said. "Our schedule speaks for itself. You look at the schedule and you can figure it out real quickly."
San Antonio has 12 games remaining, which is more than any team involved in the conference playoff race.
The Spurs also have a three-games-in-three-nights stretch starting Monday at Golden State.
"It's just crazy," Popovich said, "so I have to do something about it. It just doesn't make sense to have those guys playing four in five nights or three in a row or anything like that."
Popovich called the decision to rest his stars was "pretty much a no-brainer" because of the upcoming schedule.