Mosley displayed speed, aggressiveness and instinct, leading the Ravens with five tackles, including three solos. The former Alabama consensus All-American and Butkus Award winner also sacked 49ers backup quarterback Blaine Gabbert on a blitz where he busted through running back Jewel Hampton's attempt to block him.
"I'm not taking nothing for granted," Mosley said. "I'm an even-keel kind of guy. I'm just making my mark. We've been practicing for a long time. I finally got the first preseason game out of the way. I kind of got a chance to feel the speed, and how I needed to get downhill or certain things I need to do to make plays and just get my body right and stack plays."
Drafted 17th overall, Mosley had 319 career tackles, 6.5 sacks, five interceptions, two forced fumbles and one fumble recovery for the Crimson Tide.
Mosley is already making a good impression on his veteran teammates.
"It shows the front office made a good decision," said outside linebacker Elvis Dumervil, a three-time Pro Bowl selection. "C.J. is humble and ready to work and he's making plays. It's expected from him. This is a start. He wants to do it, and that's where it starts.
"He works hard and practices hard. It's a good thing. We need him. He's a good guy. He doesn't say much. He's very mature for his age. He's been a good teammate."
After the sack Thursday, Mosley attempted in vain to locate his parents in the stands.
"I tried to find my parents on the sideline, point them out," Mosley said. "I didn't see them. When they ask me about it, I'm going to say I saw them."
Mosley was on the field for 24 snaps, tying defensive end Kapron Lewis-Moore for the most snaps on the Ravens' defense.
"C.J. seemed like he played pretty well," coach John Harbaugh said. "He played a lot."
Since his arrival at training camp, Mosley has shown a lot of skill at diagnosing plays and anticipating what the offense is planning to do. He's fast enough to close down plays before they can get started and gets a lot of depth when retreating on his pass drops, breaking quickly on the football.
"C.J. is one of those rare, young players that has a great deal of exposure to a lot of good defense," linebackers coach Ted Monachino said. "So, the transition for him mentally hasn't been very difficult at all. The transition, physically, is one that he's got to catch up. He's got a little bit of a curve there, but has made a huge jump just in the first 10 days of practice. So, we're thrilled where he is right now."
Mosley got off to a rough start initially as 49ers rookie running back Carlos Hyde gashed the defense for 39 yards on five carries. Mosley also accepted responsibility for allowing a 17-yard reception to tight end Vance McDonald.
"The first drive, I made a mental error," Mosley said. "I let the tight end get outside on me. Besides that, I had a pretty good night. They had their scheme going, hitting some holes and making some plays. I made my adjustments on the sidelines."
Mosley acknowledged before the game that he anticipated some nervousness.
Once the game began, he appeared poised and relaxed.
"I'm a very steady guy, so I was ready to play," Mosley said. "I wasn't too tensed-up or too calm. I was at the right level."
During the game, Mosley diagnosed a screen properly. He forced an incompletion with a well-timed blitz.
Mosley has a history of adjusting quickly to a higher level of football.
He was a starter as a true freshman at Alabama and was named a freshman All-American, returning two interceptions for touchdowns. Last year, he shared Southeastern Conference Defensive Player of the Year with Missouri defensive end Michael Sam.
"Our scouts do a good job of trying to [figure] out his football IQ, and what is that exactly? Guys who really understand the game and pick things up quickly, and C.J. is one of those guys," Harbaugh said. "You don't have to tell him more than once, and he makes corrections really quickly."