Josh Rosen couldn't muster another improbable fourth quarter comeback in Week 4 as UCLA dropped its Pac-12 opener to Stanford by a 58-34 score on The Farm in Palo Alto, Calif.

It was the Cardinal quarterback situation, however, that was the story of the game as David Shaw used three signal callers to improve to 2-2 (1-1 Pac-12) on the year.

K.J. Costello stepped up after starter Keller Chryst left the game in the first half with an injury, finishing with 123 yards, a rushing score and the first two touchdown passes of the sophomore's career. Junior Ryan Burns added 13 yards via the air.

While Stanford's second-string quarterback was putting himself on the map, it was the Cardinal running game that wore down the Bruins. Bryce Love scored a touchdown and rushed for a career-best 272 of the team's 405 yards on the ground. It marked the fourth time this year that the junior rushed for 180 yards or more in a game.

Rosen, who threw four touchdown passes in the fourth quarter in the season opener against Texas A&M, connected on 40 of 60 passes for 480 yards, but a sloppy UCLA (2-2, 0-1) was its own worst enemy.

The Bruins fumbled three times, losing two of them. They had 12 penalties for 127 yards and had a kick blocked.

UCLA chipped away at its 23-13 halftime deficit. Soso Jamabo (100 yards rushing) cut into the lead with a 3-yard touchdown run early in the third quarter.

Costello and Stanford quickly answered, however, with a 15-yard strike to Trenton Irwin. Cameron Scarlett added his second of three rushing touchdowns on the night, after a UCLA fumble, to up the score to 37-20. The junior tallied 86 yards, also a career high.

Rosen kept UCLA in the game with touchdowns to Darren Andrews, Eldridge Massington and Jordan Lasley (158 receiving yards). But it wasn't enough to overcome Stanford. The Cardinal drove the final daggers into the Bruins with touchdowns by tight end Dalton Schultz and a 69-yard scamper by Love.

Officials ejected Stanford cornerback Alijah Holder for targeting early in the third quarter. The senior led the Cardinal with eight tackles prior to the ejection. Free safety Ben Edwards was flagged for targeting on the very next play, but was absolved of the personal foul upon video review. Edwards added an interception with 4:27 to play to seal the win.

UCLA safety Adarius Pickett was also ejected in the first half. The junior is the third Bruin to be ejected for targeting in as many games.