SAN DIEGO -- As an undrafted rookie, league observers questioned whether Antonio Gates could make the transition from a college basketball player out of Kent State to an NFL tight end.

After earning eight trips to the Pro Bowl, critics more recently questioned the 35-year-old tight end's ability to produce at a high level into his 30s.

Now, returning from a four-game suspension for violating the league's policy on performance-enhancing drugs, Gates said he's using people questioning his character as fuel as he returns to the field for the first time this season against the Pittsburgh Steelers on Monday Night Football.

Specifically, Gates pointed to comments made by Hall of Fame tight end Shannon Sharpe. The former Denver Broncos tight end said Gates' positive test "calls into question everything he accomplished" and "cheated the game," during an interview on SiriusXM NFL Radio.

Gates would not say what he tested positive for, other than the substance caused elevated testosterone levels that led to a positive test.

"If he does it at the beginning of his career because he was an undrafted free agent, people are gonna say he did it to get in the league," Sharpe said. "Now he did it Year 13, Year 14 -- people are going to say he did it to remain in the league. It does, it makes you question everything someone has ever accomplished."

Gates says he hasn't spoken to Sharpe since those comments in July, but he also has not forgotten them.

"We played against each other, so it wasn't like he was a teammate of mine or we were the best of friends," Gates said. "But we do know each other, and it was always competitive in terms of who was the better tight end, I do know that.

"And we joked about it. We laughed about it. So I guess if he can find a way to tarnish something I think he probably would."

With 10,014 career receiving yards, Gates needs 47 more to move past Sharpe into third in NFL history for tight ends. He needs 28 catches to move past Sharpe (815) into third all-time among tight ends.

But Gates said those numbers don't matter to him.

"I've never cared about what someone else has accomplished," Gates said. "I always go out and I just play."

In terms of his time off, Gates said he made a conscious effort not to watch any San Diego games so he could come back with a clear mind, although he did check the scores of the games through his cell phone.

Gates said his workouts focused on staying under his 255-pound playing weight; Gates acknowledged a little bit of expected rust during his first practice on Wednesday.

"It was difficult because for the first time I had no effect on the game," Gates said the first four games. "No matter what I felt, what I was able to do had no effect on the game. And normally for the last 12 years I've been able to have an effect."

Gates said his teammates welcomed him back with open arms, and he's focused on reestablishing himself as player in the league.

"Gates is a playmaker," rookie running back Melvin Gordon said. "They say Gates gets busy out there on Sundays, Mondays and Thursdays. Everybody knows Gates. So it's always good to get one of your playmakers back on the field."

Pittsburgh Steelers head coach Mike Tomlin doesn't expect San Diego's offense to change much with Gates back in the fold.

"Obviously, Antonio is a really talented player and his accomplishments speak for themselves," Tomlin said. "He doesn't need any endorsement from me. But it appears they play ball the way they play ball. It's just great to infuse a guy of his talents back into the mix."