SAN FRANCISCO -- Wide receiver A.J. Jenkins did something in the second quarter of the 49ers' exhibition opener that he did not accomplish during the entire regular season of his rookie year.
Jenkins caught a pass. But, then, he failed to hold onto it.
Jenkins, who is in a battle to earn a role on the 49ers' offense, fumbled away his one and only reception as part of the 49ers' three-turnover first half in the club's 10-6 loss to the Denver Broncos on Thursday at Candlestick Park.
Jenkins had two other passes thrown his way in the first half with notable results.
Teammate Chad Hall came back to catch a Colt McCoy pass in front of Jenkins for a 15-yard gain. Later on that drive, McCoy overthrew Jenkins on a pass that was intercepted by Broncos safety Mike Adams.
Jenkins appears to be in a battle to earn a spot on the 49ers' roster after being the team's first-round pick in the 2012 draft. Jenkins' best play came at the end of the third quarter when he flattened a Broncos cornerback on a running play.
McCoy sidelined: Neither Colt McCoy nor Scott Tolzien made a move for the 49ers' backup quarterback job. But McCoy might have taken the biggest hit when he left the game in the second quarter with what he described as a "stinger" to his right neck and shoulder area. He is scheduled to undergo an MRI on Friday morning, he said.
McCoy's last pass attempt was an interception in which his right arm appeared to be struck as he was throwing. But McCoy said the injury took place prior to that when he was hit at the end of a broken play that resulted in a run attempt. The extent of his injury was not immediately known, but it did not appear to be serious. McCoy remained on the sideline for the entire second half in full uniform, including shoulder pads.
McCoy completed just three of his seven pass attempts for 41 yards and an interception.
Moody injured: Rookie linebacker Nick Moody sustained a left knee injury in the third quarter when he was rolled up on during a running play. He was carted off the field for further observation. Moody, a sixth-round draft pick, was selected with the intention of being one of the 49ers' core special-teams players.
For starters: Colin Kaepernick played the opening drive, which consisted of 12 offensive plays. He looked sharp during his brief outing, completing 4-of-4 pass attempts for 38 yards. He also picked up a first down with a 6-yard scramble -- a play that he wisely ended with a slide to avoid additional contact.
The 49ers settled for Phil Dawson's 32-yard field goal for a 3-0 lead.
LaMichael James started at running back and saw a lot of action in the first two possessions. James carried eight times for 27 yards. The 49ers' starting offensive line played the first two series of the game.
Defensively, the 49ers went to their bench at the beginning of the second series. Broncos quarterback Peyton Manning played just one series. Denver picked up two first downs against the 49ers' defense. The series ended when Aldon Smith made a stop of running back Ronnie Hillman for a 1-yard gain and then pressured Manning into an incompletion.
Premium reserves: Nnamdi Asomugha and Tramaine Brock replaced starting cornerbacks Carlos Rogers and Tarell Brown. Asomugha had a nice hit on a Brock Osweiler completion to Gerell Robinson that netted just 5 yards.
Perrish Cox, another cornerback who is battling for a role on the team, had a solid game. He showed good burst on a kickoff return, and he also broke up two passes at the cornerback position.
Rookie report: Safety Eric Reid entered in the second quarter and looked comfortable in his NFL debut. He appeared to have a command of the defense as he regularly made pre-snap adjustments in the secondary. Reid also came through with a couple of big hits, including de-cleater on tight end Virgil Green at the end of a 13-yard play. . . Tight end Vance McDonald saw a lot of action and caught a team-high four passes for 66 yards. He also dropped a pass in the second quarter.
On the bubble: Undrafted rookie running back D.J. Harper, who opened some eyes during training camp with his performance in blitz pickup drills, had a nightmare of a first half in his first NFL exhibition game. Harper fumbled after a hit from linebacker Nate Irving in the backfield. Shaun Phillips scooped up the loose ball and returned it 9 yards for the only touchdown of the game.
Veteran Kassim Osgood helped himself out with a solid first half as a wide receiver. He caught three passes for 34 yards. Osgood's value to the 49ers is on special teams, and he made two convincing tackles on the coverage units.
Wide receiver Ricardo Lockette also showed up on special teams. He made an exceptional tackle of Broncos' return specialist Trindon Holliday at the Denver 2-yard line after a Colton Schmidt punt.
Winds of change: Dawson, who said he was unable to figure out the wind pattern of Candlestick Park during his seven or eight visits in the offseason, connected on two field goals.
But he also missed badly on a 44-yard attempt on the final play of the first half. Dawson did not appear to strike the ball well, and it drifted wide to the left. Dawson, a 15th-year player, is taking over for David Akers, who struggled mightily last season.
Sitting it out: Running back Frank Gore, linebacker Patrick Willis, defensive end Justin Smith, wide receiver Kyle Williams, wide receiver Quinton Patton, center Jonathan Goodwin.
What's next: The 49ers return to practice Saturday for a practice during Fan Fest at Candlestick Park. The 49ers' next exhibition game is Friday, Aug. 16, against Alex Smith and the Kansas City Chiefs at Arrowhead Stadium.