"We feel like when healthy, Jesse is among the best relief pitchers in the game," Friedman said. "As far as prognosis, we don't really know, in that we haven't had a chance to put our hands on him.
"It's too early to speculate on exactly when, but again, with our training staff and everything we've learned about Jesse, we're confident he will come back and impact this team."
One of the best setup relievers before his injury, Crain posted an 0.74 ERA in 36 2/3 innings this season, striking out 46 while holding batters to a .225 average. He has yet to allow a home run. He was named an All-Star for the first time this season.
When he returns, Crain will bolster a staff that has been the best in the Majors in July, going 19-4 while posting a 2.44 ERA heading into Monday's game with the Red Sox.
"We have very few off-days," Friedman said. "And for us to have a chance to reach the playoffs and be really strong in the playoffs, having another option in high-leverage situations was something that was very appealing to us.
"We have a lot of confidence in the guys we have in our bullpen and like our bullpen mix, but to keep people fresh, being able to mix things around a little bit and being able to spot up in creative ways was something that was very attractive to us. And we potentially strengthened the strength."
Friedman said compensation to the White Sox will likely be determined after the 2013 season is complete, but it won't center on Crain's performance. Friedman said the Rays have a good relationship with the White Sox and there is already a framework in place.
"We certainly think he has the ability to influence this year's pennant race very positively for the Rays," said White Sox senior vice president and general manager Rick Hahn.
Crain, a Toronto native, began his career with the Twins, serving as the closer for Triple-A Rochester before a successful seven-year career in Minnesota. He's spent the last three years with the White Sox.
Crain has four career saves and 125 holds.