In return, Texas sent pitcher Matt Harrison and five prospects -- catcher Jorge Alfaro, outfielder Nick Williams and pitchers Jake Thompson, Alec Asher and Jerad Eickhoff -- to Philadelphia.
Even before the deal was finalized, the Rangers' clubhouse accepted the trade as reality, making for a strange mood. Harrison is an eight-year veteran who is greatly respected among teammates. Hamels, though, gives the Rangers a chance to win in 2015, and an even better opportunity to contend in the future. Next season, Hamels will a healthy Yu Darvish and Derek Holland in the Rangers rotation.
"I think it's a situation where we're losing a guy a lot of us cared about," pitcher Colby Lewis said. "But at the same time, yeah, you're excited about getting another guy in here with his track record and what he's done in the game. You're just looking forward to hopefully he can continue to do that for us."
Hamels (6-7, 3.64 ERA) is a three-time National League All-Star and was the 2008 World Series MVP. He has finished in the top eight in the NL Cy Young voting four times, and his last start with the Phillies was a no-hitter against the Cubs on Saturday.
"Who [wouldn't] want to have him in the pitching rotation?" catcher Robinson Chirinos said. "It's real exciting, hopefully to have the chance to catch him."
In Diekman, the Rangers get a needed left-handed piece in the bullpen. Diekman went 2-1 with a 5.15 ERA for the Phillies in 41 appearances this season. His fastball clocks in at upward of 96 mph, and he averages 11.7 strikeouts per nine innings over his four-year Major League career.
"He throws flakes," said Rangers reliever Sam Freeman, a former Cardinal who has seen Diekman pitch. "He has a dirty slider. He beats up both lefties and righties. He has rung up a lot of dudes."
Some of the Rangers' more experienced players were subdued by the prospect of losing Harrison. But there remains an underlying sense of excitement, like gazing at a present under the tree, that comes with Hamels becoming a Ranger.
"It makes us better," outfielder Josh Hamilton said. "To get a guy like Hamels and a lefty for the 'pen, you can always use guys of that caliber."
Shortstop Elvis Andrus said the move can be a jolt for the Rangers' clubhouse, not totally unlike when the team traded for Cliff Lee in 2010.
Despite being eight games out of first place at 48-52 entering Thursday, the moves affirm hope for this season. Hamels could start for Texas as soon as Saturday. And though the Rangers are giving up a bevy of prospects, the thought of Hamels' name in the rotation in 2016 and beyond is daunting for opponents.
"For sure, if you start thinking about what's gonna happen next year when you've got Darvish and everybody healthy, it's a great rotation," Andrus said. "When you have a pitcher of the magnitude of Cole Hamels, it's always great. Believe me, I know he's going to make us a better team right away."
Cody Stavenhagen is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.