BOSTON –– The Red Sox breathed a heavy sigh of relief Saturday, upon learning that shortstop Alex Gonzalez’ wrist was fine following a terrifying moment when he was hit by a Kerry Wood pitch on Thursday.

“He got me good, but thank God it’s nothing big,” Gonzalez said. “He got me good, though. There’s only a little bit of swelling.”

Losing Gonzalez would have been crippling to the Red Sox heading into the playoffs. No position on the field has been more unsettled this season than shortstop, and the backup options –– Chris Woodward and an injured Jed Lowrie –– represented significant downgrades for a defense that has, on the whole, underachieved this season.

The Red Sox came into the season expecting to be one of the league’s better defensive teams. Only a few months in, it became clear that was not the case. Gonzalez was added in mid-August to help address the problem.

“Defense this year hasn’t been what we wanted it to be and what we expected it to be. It’s an area of focus for us trying to improve it and work around it,” said general manager Theo Epstein.

By surface-level measures, like fielding percentage, the Boston defense is pretty good: it doesn’t make a lot of errors, and the players usually make the plays they should make. The Sox are tied with several other teams for fourth in the majors in fielding percentage, with a .986 mark.

But by other measures, like defensive efficiency –– the rate at which teams convert balls put into play into outs –– they are poor, speaking to a lack of range, arm strength or fielding instincts by some players in the field.

In 2007, the Red Sox led the league in defensive efficiency, converting 70.5 percent of balls put in play into outs. This season, they are the third-worst team in the league, converting 67.9 percent of balls in play into outs.

Early in the season, it was clear the defense was not what it once was. Mike Lowell was still as sure-handed as a third baseman can be, but his extended recovery from hip surgery limited his range and his jumps. Injuries to gold-glove first baseman Kevin Youkilis forced the team to play Jeff Bailey for long stretches.

But the biggest problems came at shortstop. Early in the season, both Jed Lowrie and Julio Lugo went down to injury, forcing replacement Nick Green to learn the shortstop position on the fly. Green has speed, a strong arm, and some range, but was no replacement for a natural, seasoned shortstop.

“The shortstop position, because of the performances we had in the first half, we had a need to improve.” Epstein said. “It was also fairly easy for us to improve.”

The team needed to find another option, he said –– and not necessarily a spectacular fielder, just a steady and sure one. Enter Gonzalez, who was acquired from the Reds on Aug. 14 for cash and a minor leaguer.

“We were really well below the average, well below the average, well below where we wanted to be at shortstop defense for a significant part of the season,” Epstein continued. “And so bringing in Alex, who’s been really steady since he’s been here was a significant upgrade, in part because of how reliable he’s been, how good his hands are, how good his arm is, his instincts, but also in part because of the performance we had early, and so that’s been a steadying influence on our overall defense and our pitching staff since he’s been here.”

The stability Gonzalez brought to the Red Sox cannot be understated. For the month after his arrival, the team went 19-10. He provided steady defense, despite having several serious injuries the last few years, said manager Terry Francona.

“That’s the one thing he said when he came over. He goes, ‘I want to play.’ ‘OK.’ I have no problem running him out. He prepares every day. He takes good care of himself. There’s no reason he can’t play every day. He’s done a good job. You start getting some injuries like he had with the knee, those are pretty serious things. It’s one thing for a guy to say I want to play and then go out there and limp. But he’s taken care of himself to the point that he can go out there and be a real good player,” Francona said.

Around the same time, the Red Sox made several other defensive improvements, bringing in Casey Kotchman to play backup first base, and Victor Martinez to catch and play first.

Kotchman is excellent defensively. Martinez is known for his offense, and his ability to throw out base stealers leaves much to be desired. But as a first baseman, Martinez is underrated, and the combination of Martinez at first and Youkilis at third has been effective.

“I think he’s pretty good,” Francona said of Martinez at first base. “Very active, he’s comfortable playing off the line, so he cuts down some real estate that way –– I mean, I can see why that can get overlooked, because he’s a catcher, but he does a good job over there,” Francona said.

The additions of the trio made a difference over time in Boston’s defensive efficiency.

After two months, at the beginning of June, Boston was converting only 67.4 percent of batted balls into outs. The addition of Gonzalez, and to a lesser extent Martinez and Kotchman, has allowed the Sox to improve that number somewhat, to the final 67.9-percent figure.

Now, with Gonzalez healthy, the Red Sox don’t have to wonder who will start at shortstop in the playoffs.

The bigger question is who will play behind him. Nick Green’s back issues are preventing him from fielding his position, and it’s looking like he won’t be ready for the playoff roster. Switch-hitter Jed Lowrie’s rehab from wrist surgery is not complete, and taking multiple at-bats from the left side may cause his injury to flare up. In some situations, he might not be able to play a full game.

“It’s not that he can’t, it’s just he’s fatigues a little bit. If he went up there and walked four times, he could play 12 games in a row. There’s some area there we just don’t know. If he takes four check swings, it may grab him or it may not. We don’t know,” Francona said.

Woodward, the other option, is still in Florida with his wife following the birth of his son several days ago. It’s not clear when he will return.

“He would like to come back, but he’s not able to, yet,” Francona said. “He had the baby and they baby is O.K. There are some extenuating circumstances and he needs to be home. There’s good reason and he has our blessing to be home.”

dbarbari@projo.com