SANTA CLARA, Calif. -- After two years of doing whatever he could to fit in to the San Francisco 49ers' defense, it turned out that what linebacker Eli Harold needed to earn his chance at a starting job in the NFL was offseason surgery and a trip to Hawai'i.

On Friday, the Niners released veteran Sam linebacker Ahmad Brooks, who had been in a training camp competition with Harold. Coach Kyle Shanahan acknowledged soon after the move that Harold is stepping into the role Brooks leaves behind.

"It really comes down to the final 53," Shanahan said. "I’m starting to get a better feel for what that is now. It’s taken me some time. It’s taken [general manager] John [Lynch] and I some time because we’ve got to know what our group is like. It’s really been nothing against Ahmad. It’s, how are we going to get 53 guys on this team, the right type of 53 guys?

"You think of guys that might not be as good in Week 1, but you think they’ll be ready in Week 3. What gives you the best chance to win in Week 1 and Week 8 and Week 16? You’ve got to play all that into account. It’s rarely as simple as saying it’s the starter versus the backup versus the third-string guy. It has to do with the overall 53."

Therein lies the rub in the Niners' decision to go with Harold over Brooks. On paper, one could look at this and say it's a financially driven move, as Brooks would count $5.2 million more against this year's salary cap than Harold. But the Niners have the most salary-cap space in the league, so it's not as though that really played a part in releasing Brooks.

Instead, as Shanahan mentioned, it's about finding the player who can give the team the best option at the position for the short and long term. At 33, Brooks still has some gas left in the tank, but he is what he is at this point in his career. Harold is only 23, entering just his third NFL season, and he is playing at a position and a weight that has him feeling more comfortable than at any point in his previous two seasons.

In some ways, Harold is starting anew in new coordinator Robert Saleh's 4-3 defensive scheme. After spending most of his football life on the right side of the defense, he moved to the left and the Sam linebacker spot under the new regime. In making that transition, Harold made it a point to return healthy and in shape.

Harold had toe surgery in late January, causing him to miss part of the offseason program. The injury slowed Harold in 2016, when he was limited to 13 starts as an outside linebacker because of other injuries and managed only 37 tackles and three sacks. Harold believes the toe injury came as the result of his weight gain: He went from 245 pounds as a rookie in 2015 to playing at closer to 270 pounds in his second season.

Although that weight gain was by design in an effort to help Harold better set the edge in the run game, it didn't seem to make much difference. After traveling with teammates DeForest Buckner, Arik Armstead and Ronald Blair to work out in Hawai'i with Seattle Seahawks defensive linemen Michael Bennett and Cliff Avril, Harold realized he could get the job done without carrying the extra weight.

"[Setting the edge was] something that was talked bad about me for a long time," Harold said. "And this offseason, being with Michael Bennett and the guys in Hawai'i, I really focused on that."

For Harold, the biggest thing was understanding leverage and pad level. While he often will face blockers who carry a weight advantage, he can counter that by getting underneath their pads and establishing his position. Upon returning for training camp, Harold continued to seek the advice of veterans such as Niners defensive end Elvis Dumervil, who at 5-foot-11 knows plenty about winning with leverage.

"[I'm] just picking their brain and learning how to watch film and studying," Harold said. "Like I said, gaining all that weight last year really set me back, and I really thought it was going to help. I feel like that led to me getting hurt. But learning that I don’t need to be that heavy to be a great football player and to play this position, I am really glad we moved to a 4-3, and I feel like I’ll be successful in this defense."

Harold has moved past the foot injury and a concussion that slowed him early in camp. And with Brooks gone, Harold will have a chance to get even more reps with the starting defense. Not that he's taking anything for granted. With veterans such as Dekoda Watson still lurking in the background, Harold is aware that keeping a starting job could be as hard as earning one.

For now, the Niners seem intent on seeing if Harold can maximize his potential and offer the type of upside Brooks could no longer bring to the table.

“He’s a physical player who can set the edge," Shanahan said. "I think he’s got a lot more in him. I think he can continue to improve. But I love the way he fights. I love the way he competes every day. He’s a guy, to me, who’s trying to be his best every day. I think there’s a lot more to come. Usually guys who have that type of mentality, they usually get better, not worse.”