ST. PETERSBURG, Fla. -- On the day the Tampa Bay Rays learned starter Alex Cobb was done for the season and heading for elbow surgery, Nathan Karns showed there is still strong pitching left on the roster.

The Rays right-hander took a two-hit shutout into the eighth inning, and pinch-hitter Brandon Guyer hit a three-run home run in the sixth, lifting the Rays to an 8-2 win over the Texas Rangers at Tropicana Field on Friday.

"It's very encouraging," Rays manager Kevin Cash said of Karns' strong pitching. "We've leaned on him among others throughout the early part of the year. Now we know we're going to be leaning on him even that much more. That gave us a jolt of confidence there."

Tampa Bay (16-14) continues to get strong pitching from Karns, who had allowed one run total in his previous two starts. Guyer's big hit came on the first pitch after the Rays chased Texas starter Yovani Gallardo, who had allowed only one hit in the first five innings for the Rangers (12-17).

After clinging to a 1-0 lead heading into the sixth inning, the Rays broke the game open in the sixth inning. Center fielder Kevin Kiermaier reached on a fielding error by shortstop Elvis Andrus, and after a single by first baseman James Loney, Kiermaier scored on a single by designated hitter Evan Longoria for a 2-0 lead.

The Rangers went to the bullpen and brought in left-hander Alex Claudio, who had earned his first major-league win in relief Thursday night. The Rays countered with pinch-hitter Guyer, who hit the first pitch for a three-run home run and a 5-0 lead. It was Guyer's second home run this season and the Rays' third pinch-hit home run of the year.

"He tried to throw a sinker down and away and left it pretty much center-cut," Rangers manager Jeff Banister said. "He got barrel on it and put it out of the ballpark. Really the two home runs we gave up were the difference in the game."

Texas threatened in the eighth, with a solo home run by second baseman Adam Rosales, then three straight singles to load the bases with no outs. But Rays reliever Xavier Cedeno struck out designated hitter Prince Fielder, and reliever Kevin Jepsen got the last two outs, the only damage coming on a sacrifice fly by third baseman Adrian Beltre. Jepsen struck out left fielder Carlos Peguero with the bases loaded to end the inning.

"Our guys are going to continue to battle," Banister said. "They feel like they're in every game no matter what the score is and they're going to continue to get after it. We'll get back after them again tomorrow night."

The Rays added three runs in the ninth, with three straight singles, a sacrifice fly by second baseman Tim Beckham, then a two-out RBI single by rookie third baseman Jake Elmore, with a second run scoring on a throwing error.

With Cobb headed for Tommy John surgery and done until at least late in the 2016 season, the Rays will need unproven starters like Karns to step up into larger roles. The Rays have stayed above .500 despite missing key starting pitchers for the first month of the season, and now much longer for Cobb.

Cobb was moved to the 60-day disabled list, and the Rays had to put left-hander Drew Smyly back on the 15-day DL with stiffness in his throwing shoulder. They called up Matt Andriese to be Saturday's starter, moving right-hander Jake Odorizzi back to Sunday's game as a result. The Rays also recalled right-handed reliever Andrew Bellatti from Durham to join the 25-man roster and sent Elmore down.

The starting pitchers -- Gallardo and Karns -- came in with matching 4.05 ERAs and both dominated in the first five innings.

Gallardo gave up one hit -- a solo home run to right in the second inning by Rays left fielder David DeJesus -- but was otherwise in control.

Karns allowed two hits in the first six innings -- singles by center fielder Delino DeShields Jr. and Fielder -- but kept the Rangers off the scoreboard, thanks in part to nine strikeouts.

Karns threw just seven pitches in the sixth inning, coming through for an overworked bullpen that pitched five-plus innings the previous night in relief of Chris Archer.

"I apologized for (pulling him in his last start after) 60 pitches, so I made it up to him tonight and got him (117)," Cash said. "It worked out well. ... What a job he did to dial it in and get us to where he did."

NOTES: Alex Cobb, who hasn't pitched this season because of right forearm tendinitis, will undergo Tommy John surgery next week and miss all of 2015 and most of the 2016 season. Cobb, who would have been the Rays' Opening-Day starter if healthy, had considered alternatives to surgery after an MRI revealed a tear in his elbow. He's hoping to return late next season. ... Tampa Bay has 11 players on the disabled list, including seven pitchers. ... Rays 3B Evan Longoria saw his major league-best streaks of 198 consecutive games started and 270 consecutive games played end when he sat out Thursday's game because of flu-like symptoms. Longoria was back in the lineup Friday and batting cleanup. ... According to Elias Sports Bureau, Rangers OF Delino DeShields Jr. became just the second rookie in 23 seasons to have three walks and three steals in the same game on Thursday against the Rays. The other was Pirates OF Andrew McCutchen in 2009.