It forced Haren to exit after just 4 2/3 innings and the Angels' bullpen struggled to pick up the slack in a 6-3 loss to the Red Sox at Angel Stadium.
"I couldn't believe it," Angels center fielder Torii Hunter said. "We've had a lot of bad luck, and we trade for a new guy and, bam, he gets hit on the forearm. It's just the way it's been going for us, but fortunately he's doing well right now, so we'll see how sore he is tomorrow."
The good news for the Angels is that the injury was initially diagnosed as a contusion. Haren is scheduled to be taken in for X-rays by team orthopedist Lewis Yocum on Tuesday.
"It's a little sore, but nothing throbbing," Haren said. "There's nothing that I'm worried about. Hopefully I'll get a decent night's rest and wake up and it won't be so bad. I'll get some treatment, and I'll be out there soon."
But it was still an especially strange situation for Haren, not only because it came in his first start with the Angels, but because he was acquired for his durability, as he's made at least 33 starts and thrown at least 216 innings in each of the last five seasons.
Angels manager Mike Scioscia, however, sounded optimistic about Haren's recovery, saying, "It's not out of the realm of possibility" that he'll make his next scheduled start in the rotation.
"Sometimes these things heal quickly," Scioscia said. "It doesn't seem like it got much of the bone and just some soft tissue. But we'll let Dr. Yocum and the medical staff make that decision to see what the extent might be."
Haren was actually pitching effectively before that injury, as he'd allowed two runs on seven hits with eight strikeouts and no walks allowed. He gave up a run in the second inning on a single from Victor Martinez after Adrian Beltre's triple off the glove of Hunter in deep center field, before allowing another run in the third on a homer from David Ortiz.
"He threw the ball well," said Red Sox right fielder J.D. Drew. "He was mixing his pitches well, had some good sink going, throwing that nasty split. He, for sure, had me off-balance. I don't think many people have you off-balance."
Haren even wanted to stay in the game despite being hit on the forearm but was advised by team trainer Rick Smith and the Angels' coaching staff to come out of the game.
"It hurt, but I didn't feel it hit any bone so that was a good thing," Haren said. "But it was a little bit of a scare. I would've liked to chance to get a throw, but they told me it would probably be best if I called it a day."
As for the Angels' offense, they struck early with a first-inning home run from Bobby Abreu off Clay Buchholz, but the Sox right-hander was otherwise brilliant, allowing just one run over seven innings to pick up the win.
The Angels had a chance to put some runs on the board in the second inning, when they loaded the bases with no outs, but Juan Rivera and Jeff Mathis both popped out to Youkilis at first base before Erick Aybar flew out to left field to end the inning.
"We had some opportunities there," Scioscia said. "You have to give Buchholz some credit there. He made some big pitches and got out of the inning and settled in to pitch deep into the game."
The Red Sox even added two more runs in the eighth inning against Fernando Rodney on a second homer from Ortiz, who won the State Farm Home Run Derby at Angel Stadium earlier this month.
But the Angels came within a run of the Red Sox in the bottom of the frame with a two-run shot from Hideki Matsui off Scott Atchison, with the homer being upheld after a two-minute review by the umpiring crew.
The Red Sox struck quickly with two more insurance runs, however, as another Angels right-hander had a tough debut when Michael Kohn allowed a two-run double to Drew to put the game out of reach for the Angels.
Scioscia defended his decision to bring in Kohn for his first big league outing despite such a tight game against a top offensive club, saying Kohn was "brought up to pitch" despite the tough situation.
"We have confidence any of our guys are going to keep us in the game and that's what we felt about Michael," Scioscia said. "He has a strong arm and is deceptive in his delivery but in his first time he was probably a bit jittery. His fastball was up a little bit, but his stuff is going to play in the big leagues. He just didn't get done tonight."