NAPA — DeMarcus Van Dyke has gone from phenom to flame out in the course of 18 practice sessions.

As the Raiders concluded their training camp Thursday, the realization is that the true worth of the second-year cornerback out of Miami lies somewhere in between.

Van Dyke’s recent struggles suggest he isn’t yet the playmaker who wowed teammates and coaches by breaking up numerous passes while subbing for injured starter Ron Bartell during the early days of training camp.

“I’ve just got to go out there and play ball,”Van Dyke said. “I’m doing too much thinking, I think. Coach (Jason) Tarver gave me a lot of advice, telling me to go out and play ball. Don’t worry about making mistakes, just play like I was the first two weeks of camp.”

As July turned to August, Van Dyke looked to be pushing for a starting job, perhaps at Shawntae Spencer’s spot, once Bartell returned.

Van Dyke had put on weight — he was up to 187 pounds after playing at a spindly 175 as a rookie — and was playing with confidence.

But Van Dyke is prone to being too hard on himself, sometimes lacking the short-term memory necessary to the position.

“I’m doing too much thinking about trying to make a play, just pressing too much,”Van Dyke said.

Tarver, the Raiders defensive coordinator, is hoping Van Dyke rises to the occasion against the Detroit Lions Saturday at 4 p.m. The opposition includes wide receiver Calvin Johnson, the freakishly gifted wide receiver.

“I would really like to see him play like he plays out here and just play with what we know he can do,” Tarver said. “He’s heard that from me this week. Play like he plays at practice. Because he plays pretty good at practice.”

Spencer said he thought the change in Van Dyke came when Bartell returned and Van Dyke rejoined the second team.

“He was playing so well, and then Ron came back and everything changed,”Spencer said. “He wasn’t as aggressive. He was like, ‘All right, Ron’s the starter.’ I told him, ‘No matter where you are on the depth chart, always look at yourself as a starter. Practice like a starter, prepare like a starter.”