Sunday's game against the Los Angeles Rams was viewed as a measuring stick for this San Francisco 49ers team. Sure, it was 4-0 heading into the game, but many questioned who it had beaten.

This was different. This was the defending NFC champion Rams, the team that has finished atop the NFC West the past two seasons. This was going to be the first true test for the 49ers.

They passed the test.

San Francisco is now 5-0 and is one of just two remaining undefeated teams. It is for real and led by a disruptive defense. Defensive tackle DeForest Buckner might be the heart of that defense. He is part of a defensive line that boasts five former first-round picks and held one of the top-scoring offenses to just seven points on Sunday.

After the 49ers won the game 20-7, Steve Wyche of NFL Network asked Buckner who the 49ers are.

"Underdogs, man," Buckner replied. "I mean, we've been counted out ever since day one. We're 5-0, and we just don't let the outside talk get to us. We know what we're capable of, and everybody knows now."

Buckner's defense held the Rams offense to just 157 total yards and quarterback Jared Goff to 78 yards passing. What does a performance like that say about the 49ers defense?

"We're relentless," Buckner responded. "The guys are flying around, everybody's making plays, and we've just got to come out each and every day ready to work and ready to get better."