Harold, the team's third-round draft pick, had been taking snaps during training camp this month at outside linebacker behind Aldon Smith, one of the NFL's premier edge pass rushers.
But Smith was released by the 49ers on Friday morning, leaving Harold with an accelerated learning curve and revised expectations that he may need to contribute quickly.
''I was initially shocked,'' Harold said Sunday. ''I was learning a lot from that guy. In my eyes, he was the best pass rusher in the NFL. It's just unfortunate. We lost a gem. He was a guy I definitely looked to in all the time I was here.''
The 49ers felt they found a gem in Harold when he slipped to them in the draft after a dominant junior season at Virginia. They selected him even though they already were stocked at outside linebacker with Smith, two-time Pro Bowler Ahmad Brooks and 2014 rookie Aaron Lynch. Brooks and Lynch tied for the team lead with six sacks last season, when Smith missed nine games because of an NFL suspension.
Brooks and Lynch are competing for playing time at left outside linebacker, leaving Harold and holdover veteran Corey Lemonier to fill the void left by Smith on the right side.
First-year 49ers defensive coordinator Eric Mangini believes the talent and flexibility at the position that has been bolstered by Harold can help compensate for the loss of Smith's talent, which produced 42 sacks in his first 43 games.
''A guy like Aldon, it's not a guy that you just can replace,'' Mangini said. ''He's got a unique skill set. But one of the things we focus on defensively is building flexibility in terms of what we can play and who's going to play there.
''We're moving those parts around to play left and right so we wouldn't have to play guys in the same spot the whole time. We can't easily replace someone of (Smith's) caliber, but we'll find ways collectively to get that done and it's a great opportunity for guys who may have had more limited roles had he been here.''
Harold is one of those guys, and Mangini was quick to praise the rookie when asked about his progress.
''Eli's shown some real promise here early,'' Mangini said. ''He's got a tremendous motor and he's intensely competitive. He's one of those guys that you want to tell them to hurry, but don't rush because he's so quick off the ball and so quick to read and react.''
Harold's speed and intensity already were a topic of conversation during the first week of camp before Smith's arrest. Harold also made an impression Sunday, when the 49ers practiced in full pads and had live contact during goal-line team drills.
Harold was in the middle of it all.
''Eli Harold, he came in beeping,'' 49ers coach Jim Tomsula said. ''Rookie going through it, we've all seen it, the ups and downs. But he hasn't had the downs. Eli is an energetic guy and he's doing a really good job. I think everybody sees what we saw in him and why we drafted him. He's one really athletic, competitive individual.''
And he's just getting started. With Smith gone, Harold is taking full advantage of all his other resources. After Sunday's practice, he huddled with offensive tackle Joe Staley to get the veteran's perspective on pass rushing from the other side, prompting Staley to say, ''we're excited about the way he approaches the game.''
Harold also is eager to get more pointers from former 49ers pass rusher Charles Haley, who was inducted Saturday into the Pro Football Hall of Fame. Haley worked with both Smith and Harold at the 49ers training complex in May, teaching both about some of the finer details of excelling from the defensive edge.
''I anticipate him coming back again because I really could use his help,'' Harold said.
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