Aldon Smith wore a big grin beneath his facemask, even when linebackers coach Jim Leavitt good-naturedly let him have it after the ball didn't wind up in his hands during one drill.

"Aldon! You won't be able to sleep tonight!" Leavitt hollered as Aaron Lynch patted his teammate on the helmet.

At his best, Smith is one of the NFL's most menacing sack masters who instill fear into opposing quarterbacks.

After nine games on the sideline and 14 missed the past two seasons, it's unclear how long it might take for the San Francisco star to regain his fearsome top form on one of the league's stingiest defenses.

Smith believes he will be back doing his thing in a hurry following a nine-game suspension by the league for violations of the NFL's substance-abuse and personal-conduct policies.

"You keep hearing your name, you're like 'Oh, man,' obviously I'm missed out there," Smith said. "It just put that much more in my drive when I was preparing, whether if it was with my working out or whatever I was doing to mentally be prepared to come back. It's just something I put in the back of my head."

Smith is set to make his 2014 debut Sunday at the New York Giants in some capacity, though even coach Jim Harbaugh is unsure exactly what the linebacker will bring.

"That's a good question. He did participate in training camp 11 weeks ago," Harbaugh said Wednesday. "I would refer back to last year, he missed five weeks and was able to come back and play snaps his first week back. So, good question. We'll see, take it day by day."

The addition of Smith means a lot during a tough week when linebacker Patrick Willis was lost for the season to an injured left toe that will require surgery.

Smith has been attending meetings and working out at team headquarters, but wasn't allowed to practice during his suspension.

The Giants are preparing for Smith to make plays.

"We know that he has been in the building and worked in the conditioning area. I would expect he's a player who has been in on all the meetings and conditioned himself very, very well," Giants coach Tom Coughlin said.

"I'm sure that they're just not going to throw him out there in his first game back, but I would imagine that he would have a fixed amount of plays and he may very well be in there as part of the third-down package."

Smith is enjoying all of the little things about being part of a team again — including his high-energy position coach, Leavitt.

"Geez, you guys look good! Our numbers are growing," Leavitt cheered. "Quick feet, Aldon!"

Quarterback Colin Kaepernick looked forward to seeing No. 99 across the way on the practice field, and in uniform come Sunday.

"A big lift. He's a freak of nature on the field," Kaepernick said. "He's a huge component to our defense, so very happy to have him back."

The 25-year-old Smith, one of the NFL's top pass rushers, last fall underwent treatment at an inpatient facility following his DUI arrest on Sept. 20, 2013. Last November, he pleaded not guilty to three felony counts of illegal possession of an assault weapon, stemming from a June 2012 party at his home.

In the spring, the 49ers picked up his 2015 contract option despite his legal troubles. Smith was arrested April 13 at Los Angeles International Airport. Police said Smith was randomly selected for a secondary screening and became uncooperative with the process, telling a TSA agent that he had a bomb. No charges were filed.

"There's the joy of being around the team and training and meeting and you miss the joy of practice," Harbaugh said.

Linebacker Michael Wilhoite figures Smith is "ready to wreak havoc."

"He's been here but he's been quiet. He's done what he needs to do and goes home," Wilhoite said. "When you take nine games away from a guy, that's like taking away your job for nine of the 12 months."

Smith has enjoyed doing community service and spending time with family and his young son.

For those wondering whether he can stay on the right path now, Smith plans to do his part to earn back the trust of his teammates, coaches and fans.

"I'm not going to sit here and give you some crazy answer," Smith said. "Time will show."

Notes: LB NaVorro Bowman was scheduled for an MRI on his surgically repaired left knee to better determine whether he will soon return to practice and perhaps play down the stretch this season.

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AP freelance writer Rick Eymer contributed to this report.

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