Because of a special teams turnover and a defense that was unable to get any stops in the second half, the 49ers had the ball for just 7:22 and 14 plays after halftime in their 23-13 loss to the Arizona Cardinals.
"I'll assess him on the first half because the second half is difficult to do. In the first half, I thought with the exception of the turnover we moved the ball," Nolan said. "He showed his playmaking ability, keeping plays alive. I thought that was good."
O'Sullivan finished 14-for-20 for 195 yards with an interception and two lost fumbles in his Niners debut. He was able to move the ball effectively at times, but committed three of San Francisco's five turnovers on the day to mar his first career start.
"It goes hand in hand. You can't cherry pick your performance," he said. "If you want to win in this league you can't turn the ball over that amount of times."
O'Sullivan, with his eighth NFL organization in seven seasons, had never thrown a pass in the NFL until last season with Detroit. He signed with the 49ers in the offseason and ended up beating out former No. 1 overall pick Alex Smith for the starting job.
But there was little for him to feel good about in the opener "I'm disappointed with the loss," he said. "It's tough to get past that right now. My individual performance is tied to the team so that's what I think of that."
He got few chances in the second half.
The Cardinals started with the ball in the second half and went on a 15-play drive that was capped by a 31-yard field goal by Neil Rackers that gave them a 13-10 lead. The drive consumed 6:35 of the quarter.
Then on the ensuing kickoff, Rackers pooched a short kick that linebacker Takeo Spikes bobbled at the San Francisco 31. Matt Ware came racing down field and recovered the loose ball at the 33, giving the Cardinals another possession.
They drove eight plays before scoring on Tim Hightower's short, fourth-down run that made it 20-10.
By the time the Niners ran their first play of the second half, only 3:57 remained in the quarter. O'Sullivan then engineered a 60-yard drive that consumed 6:58 and ended in Joe Nedney's 30-yard field goal.
Arizona responded with an 18-play drive that included two successful third-down conversions and a 2-yard run by Tim Hightower on fourth-and-1. The drive took 10:08 off the clock and ended in a 30-yard field goal by Rackers.
On the first play of the next drive, Travis LaBoy sacked O'Sullivan and recovered a fumble that sealed the victory.
Kurt Warner kneeled down four times, the only drive San Francisco's defense didn't allow a point on in the second half, and the 49ers only had time for one last play, a 22-yard dumpoff to Frank Gore.