SANTA CLARA, Calif. — Greg Manusky is looking for mirror images.

The San Francisco 49ers defensive coordinator said Wednesday that at positions like safety and linebacker, the ideal scenario is to feature players who are so similar in ability that opposing offenses never quite know what role each player has on a given snap.

"The best thing in this league nowadays is disguise because the quarterbacks are so well-versed in the coverages, especially during the film sessions," Manusky said.

In San Francisco, for example, there are no "strong" and "free" safeties. Both safeties are expected to be able to help out with deep coverage as well as run support, and their roles can change from one play to the next.

That's why 49ers coaches are intent on improving second-round draft pick Taylor Mays' coverage skills this offseason.

They know that at 6-foot-3, 230 pounds, Mays has the bulk and athleticism to play close to the line of scrimmage. But they also want him to be able to guard wide receivers downfield. The 49ers hope that Mays and Dashon Goldson will be their safeties for the future. They feel the duo will be more effective if both players are multi-dimensional.

Manusky also joked that he'd like "two Patrick Willises" at inside linebacker.

The 49ers, however, did draft a player, Navorro Bowman, who is nearly identical to Willis in size and appears to be just as fast. Bowman mostly played outside linebacker in Penn State's 4-3 defense and is going through a crash course at inside linebacker in the 49ers' 3-4 scheme.

Bowman said he's usually at the team facility for six or seven hours. Then it's a quick nap in his hotel room before another three hours studying the defensive playbook.

"It's going to take that kind of discipline to get on the field as fast as I want to," Bowman said. "And that's my goal to get on the field."

Et cetera — Quarterback Alex Smith's top three receivers in Wednesday's OTA practice were Ted Ginn Jr., Dominique Zeigler and rookie Kyle Williams. That's because most of the other receivers — Michael Crabtree, Josh Morgan, Jason Hill and Brandon Jones — had been tapped for random drug testing in the morning and missed most of the session.

Tight ends Vernon Davis and Delanie Walker also were tested. Joe Jon Finley, Tony Curtis and rookie Nate Byham filled in at the position.

One of the best offseason battles is at punt returner where Ginn, Williams, Jones and rookie LeRoy Vann will be competing. On Wednesday, however, all four players struggled to catch the ball in a swirling wind. Said Williams, a former center fielder: "In baseball, (the ball) doesn't really jump around like that."

The 49ers are giving David Baas and Tony Wragge plenty of work at center during practice. With rookie Mike Iupati expected to start at left guard this season, being versatile is the best way for Baas and Wragge — who have primarily been guards during their careers — to hold onto roster spots.