Hamels, officially acquired by the Rangers on Friday afternoon, arrived at Globe Life Park and entered the team's dugout in the fourth inning, while Texas was batting.
Or, maybe, he was mad because Rougned Odor made a hard slide into second base to break up a double play.
Bumgarner directed some angry words toward the Rangers. Both dugouts emptied, and Hamels was among those loitering on the field, while umpires kept the situation from escalating.
"That's some serious excitement," Hamels said of the dugouts emptying.
"To be welcomed with them and then to join them, I guess that's the type of excitement I love. It's going to be fun.
"I have a lot of catching up to do, but with this group of guys I think it's going to be pretty easy because they're true competitors, and it's fun to watch them on the field and know that those are guys that are going to be behind me."
No more incidents occurred as the Rangers beat the Phillies, 6-3, which allowed Hamels to join the postgame victory handshake line for the first time.
Texas general manager Jon Daniels paid a high price to acquire Hamels, a three-time All-Star, as well as left-handed reliever Jake Diekman, who has 49 strikeouts in 36 2/3 innings this season.
Diekman struck out 100 batters in 71 innings last season with the Phillies, and he's controllable through 2019.
The Rangers sent starter Matt Harrison and minor league prospects Jorge Alfaro (catcher), Nick Williams (outfielder), Jake Thompson (pitcher), Alec Asher (pitcher) and Jerad Eickhoff (pitcher) to the Phillies.
The complicated deal was finalized Friday. Harrison was placed on the DL with back inflammation.
As part of the trade, the Phillies are kicking in $9.5 million of the $82.1 million left on Hamels' contract and will also take on all of the nearly $33 million remaining on the contract of Harrison, sources previously told ESPN.com's Jayson Stark.
In a separate deal just before Friday's trade deadline, the Rangers acquired right-handed reliever Sam Dyson from the Miami Marlins for catcher Tomas Telis and left-handed reliever Cody Ege. Dyson, a power pitcher, is 3-3 record with a 3.68 ERA in 44 appearances and is controllable through 2021.
Hamels, who will start for Texas on Saturday against the Giants, has three years and a team option for a fourth remaining on his seven-year, $159 million contract.
Manager Jeff Banister said he never considered giving Hamels an extra day off, considering the whirlwind nature of the trade.
"I try to listen to the player and what he wants to do," Banister said. "You listen to how prepared he is and ready to go and how excited he is to do things for his teammates.
"You know he's prepared. You know he's done his work because he's a true professional. You listen to his words. He's ready to go."
The Rangers envision a future rotation of Hamels, Yu Darvish, Derek Holland and Martin Perez, with several good options competing for the fifth spot. Darvish is recovering from Tommy John surgery. Holland is rehabbing a shoulder injury.
"You go to a new team, and it's kind of like you've moved up somewhere," Hamels told reporters in Philadelphia earlier in the day. "Any time you're moving up in the minor leagues you were on to a new team, new excitement, new people. I guess that's kind of how you have to look at it. It's the only other experience I can describe.
"It's kind of like an opening day, just on a different team. I'll be happy enough that my family will be there and be a part of it because it is, it's a big journey for all of us. When I step in between those lines, it's 'game on' like I've done in the past and like I'll continue to do every fifth day that I possibly can."
Hamels concedes nothing.
He was a member of the 2007 Phillies that were seven games back on Sept.12 and won the NL East, and the 2008 Phillies, who rallied from 3 1/2 games back on Sept. 10 to win the division.
"You can never count out the work ethic, belief and talent they have here in Texas," Hamels said. "I'm just excited for the opportunity because that's what has propelled me to get to the level that I'm at."
Hamels, 31, is 6-7 with a 3.68 ERA this season with 137 strikeouts in 128 2/3 innings. He threw a no-hitter in his last start.
Daniels watched that performance in an Anaheim, California, hotel room.
"No offense to Cole -- I was looking for a couple of runs or maybe a bloop hit from the standpoint of trying to make a deal happen," Daniels said with a chuckle. "That didn't have a big effect on how we valued him.