If he was rattled by death threats, he didn’t let on. If he was hurting physically, he wouldn’t acknowledge it.

Instead, Kyle Williams stood at his locker Monday morning and calmly accepted blame for the two botched punt returns that may have cost the 49ers a trip to the Super Bowl.

“You have to take full responsibility for it, which I do,” he said, engulfed by reporters at 49ers headquarters in Santa Clara. “It’s something I made a mistake on. I’ll move through it. I promise you that.”

In a 20-17 loss in the NFC title game at Candlestick Park on Sunday, the second-year player from Arizona State bumbled twice on special teams. A ball glanced off his knee in the fourth quarter (a ruling Williams still disputes), setting up a Giants touchdown.

Then, in overtime, he fumbled a punt deep in 49ers territory that set up New York’s winning field goal.

Williams was bombarded by vicious messages on Twitter, including some death threats directed at him and his family. (Williams is single with no children).

Williams on Monday said he never even saw them.

“You know what? I really didn’t pay attention to Twitter. All the feedback I needed was family and friends, the guys in the locker room,” he said.

“That was all I needed because that’s really all that matters when you come down to it — the guys who are wearing the same jersey and the family and friends who were close by.”

But the Internet vitriol directed at his son riled Kenny Williams, the general manager of the Chicago White Sox. Kenny Williams told ESPNchicago.com that having to hear about threats on his son “makes you question our culture of sports as it stands.”

Kenny Williams also said that Kyle sustained a separated left shoulder in the third quarter against the Giants. He didn’t use that as an excuse, however — and neither did his son.

“In football, everybody is going to be dinged up here and there,” Kyle Williams said. “If (my dad) said was that it was the cause for any of the mistakes that were made, that’s not it.

“I take full responsibility for the mistake that I did make. You have to play with what you got. I was dinged up. Everybody was dinged up. That’s just football. Nobody is going to be 100 percent.”

Elsewhere in the locker room, several 49ers players expressed support for their embattled teammate. No one spoke more passionately in Williams’ defense than kicker David Akers, who missed a pair of field goals for the Philadelphia Eagles in a 21-16 playoff loss to the Green Bay Packers last year.

Akers sent a message via Twitter asking fans to back off.

“I’m irritated with the way people are treating him, absolutely,” Akers said Monday. “I think it’s ridiculous. You know, get a grip on what life’s about.

“(Williams) went out and he put his soul out there. That’s what he does. He was not trying to do anything other than make an incredible play for this team. He had a great kickoff return. Mistakes happen. We all make mistakes.”

Pro Bowl linebacker Patrick Willis, like Akers, was horrified by the backlash against Williams.

“Some of the stuff out there that I’ve seen, man, I was just like, ‘They’re saying that because they’re hurting,”’ Willis said. “But we live this game. We breathe this game. We sleep this game. If they feel that way, you can only imagine how bad he feels.

“You never want anyone to go through anything like that. It’s just unfortunate that it happened to a player like him. To me, he’s one of the best skill players in this game. To go through something like this, it probably hurts now, but it’ll only make him stronger in the long run. He has my full support.”

Williams handed punt return duties on Sunday only because Ted Ginn Jr. was inactive because of a knee injury. But Williams had plenty of experience as a returner in college.

In fact, Williams had been in the similar situation — right down to unhappy outcome. On the last play of his collegiate career at Arizona State, he fumbled a punt that his archrival Arizona recovered, and the Sun Devils lost 20-17.

But there wasn’t a berth in the Super Bowl on the line there. The high stakes explain why Williams stayed flat on the ground for several seconds after his final, fateful fumble.

“It’s one of those feelings that you don’t ever want to feel on the football field,” Williams said. “It’s painful. We’re very passionate about what we do and we’re very passionate about getting to the Super Bowl.

“To be able to be that close and not to get to it is painful, but hopefully we’re going to get through it as a team and we’ll be back.”

Cam Inman and Steve Corkran of the Bay Area News Group contributed to this report