CHICAGO -- Bears rookie linebacker Jon Bostic was fined $21,000 Wednesday for his hit on San Diego Chargers wide receiver Mike Willie.

A source familiar with the situation confirmed the fine.

The rule in question involved Bostic lowering his head and making forcible contact with a defenseless player's body, the source said. This wasn't a violation of the new "crown of the helmet rule," according to the source.

"I'm still trying to figure that stuff out," Bostic said. "Obviously I'm new to the league. There's going to be a lot of new things I'm going to have to find out. I'm learning from the older guys and really just finding out the process. I'm really just figuring out all of this stuff."

Bears' first-year head coach Marc Trestman said he thought it was a clean hit.

"I don't have any reaction right now about anything," Trestman said. "When I looked at it, other than to say that I thought it was a clean hit, his head was up and he hit with the shoulder from my vantage point on the field, and upstairs looking at it on the tape I thought it was a clean hit. That's all I need to say about it."

Bears starting weakside linebacker Lance Briggs tweeted about the fine Wednesday morning.

Shaking my head moment. Nfl fines jon bostic 21k for his clean hit against the chargers. — Lance Briggs (@LanceBriggs) August 21, 2013

Briggs went on to tweet, "Bostic's hit illegal. Hit on Dustin Keller. Legal."

Keller, a tight end for the Miami Dolphins, suffered a season-ending knee injury on a low tackle by Houston Texans safety D.J. Swearinger. The hit is legal by NFL standards.

"The league thought that (Bostic's hit) was an illegal hit; I disagree," Briggs said after practice. "I think that if you ask around the league, probably 100 percent of the league's going to say it was a legal hit, but it's not my call.