By Michael Hurley, CBS Boston

BOSTON (CBS) — The Red Sox may be the defending world champions of baseball, but it is a newcomer who might have been the biggest story on opening day.

Grady Sizemore already was the story of spring training, as the 31-year-old outfielder worked his way back from a series of serious injuries over the past few years. He had elbow surgery in 2009, microfracture surgery on his knee in 2010, sports hernia surgery in 2011 and back surgery in 2012. Prior to this spring, he hadn’t stepped on the field in a professional baseball game since September 2011.

Yet he excelled in the spring and stayed healthy, which was enough to earn the starting center fielder’s job for opening day. And when that day came on Monday, Sizemore delivered in a way few could have expected.

Sizemore strode to the plate in the top of the second inning in Baltimore with a runner on base and nobody out. He patiently took three balls to get ahead in the count before eventually smoking a 3-1 offering into right field for a single. It was his first hit — and plate appearance — since Sept. 22, 2011.

While that alone may have already made the day a success for Sizemore, he wasn’t finished yet.

Sizemore led off the fourth inning and again got ahead in the count, 3-1. He then turned on a Chris Tillman cutter on the inner half of the plate and launched it deep to right field. With a swirling wind of 20-plus mph and March temperatures that aren’t conducive to baseballs flying over fences, Sizemore still got enough of this one to coax it just over the high right-field fence of Camden Yards.

The ball bounced off the railing and back onto the field, leading to Sizemore sprinting for a triple, but the umpires quickly ruled it a home run. Just like that, Sizemore slowed down to a trot, the game was tied at 1-1 and he had his first homer since July 15, 2011.

“I think every day I’ve been here since spring training has been gratifying,” said Sizemore. “I couldn’t wait to get to the ballpark, I was up first thing in the morning. I definitely had a better appreciation just for the game and the little things that go into it.”

As the only Red Sox player to cross the plate and drive in a run on Monday, Sizemore certainly impressed his teammates.

“He was confident out there. You could see him running around and not worrying about anything, just playing the game. He’s going to be big for us,” Mike Napoli said of Sizemore. “It’s pretty incredible. But he showed signs in spring training that he can do that, so it’s one of the reasons he made the club.

“He’s got a lot of talent. If that guy stays healthy, he can be a big part of this team.”

Sizemore finished the day 2-for-4, and he saw 21 pitches in those four at-bats, which was tied for third-most on the team and more than any Orioles batter saw in the game.

Based on the comments of manager John Farrell, it seems as though the team has already moved on from “the story” of Sizemore and sees him as another ballplayer.

“I think we’re all competitors, so we’ve kind of forgotten the story line as we’ve started the regular season here, with what Grady’s gone through,” Farrell said. “He’s one of us. We’re going to be reminded of it with stories that are written, but to see the way he’s played all spring training, it’s not completely surprising he did what he did today.”

With anything that happens on opening day, there are obvious caveats. Namely, a good day in March doesn’t mean much in predicting the future. Last year, Jackie Bradley Jr. looked like a bona fide big leaguer on opening day, and he ended up in Triple-A by the end of April. And it wasn’t all sunshine for Sizemore on Monday, as he struck out swinging at a pitch in the dirt with the tying run on second base in the top of the eighth inning, part of a team-wide issue of a failure to hit with runners in scoring position which persisted all day long. The Red Sox went 0-for-10 with runners in scoring position to strand 12 runners on the day, so Sizemore was hardly the only problem there in the 2-1 Boston loss.

But there will be plenty of time this season to focus on all the negatives of every Red Sox player, and that will include Sizemore so long as he can stay healthy. That will no doubt be a positive sign of development, a process that continued to progress in the right direction on Monday.

Read more from Michael Hurley by clicking here, or find him on Twitter @michaelFhurley.