Justin Herbert's a top-tier NFL quarterback, but he actually showed off his defensive skills Friday night -- fending off a basketball headed right for his GF Madison Beers at the Lakers game!

The Chargers QB and his pop star girlfriend watched the Lakers take on the Minnesota Timberwolves from a pair of courtside seats at Crypto.com Arena ... when a ball came off the court straight at them.

While Madison cowered away from the ball, her man leaped into action ... throwing out a hand like a defensive back breaking up a pass to a wide receiver and swatting the ball away.

While Beer clearly isn't going to the WNBA anytime soon, she did look great in a revealing black tank top while she sat next to her man ... fresh off his team's Thursday night drubbing of the Minnesota Vikings.

Herbert and Beer are one of the hottest new celebrity couples -- confirming their relationship back in August -- though they were far from the only bold-faced names in attendance at Friday night's Western Conference matchup.

Dwight Howard, David Geffen, Jimmy Iovine and Corey Gamble all watched the game ... as did LeBron James, who is still sidelined with an injury and hasn't played in either of the Lakers' first two contests this season.

Halloween's right around the corner ... and Justin might want to go as a knight -- 'cause he looked heroic saving his damsel in distress from a face full of basketball!