Defense gets right back on fast track

USC’s defense is in a transitional phase, the starting linebacker corps and several linemen from last season now preparing for the NFL draft.

But after one-third of spring practice, it’s clear the unit does not seem to have lost much, if any, speed.

“The whole group can run and they’re showing it,” Coach Pete Carroll said after the defense mostly dominated during a 63-play scrimmage Sunday.

Linebacker Malcolm Smith displayed his burst after scooping up a fumble. The junior jetted up the sideline and stiff-armed quarterback Mitch Mustain to the ground on a 70-yard touchdown return.

Later, cornerback Kevin Thomas picked off a Matt Barkley pass in the end zone and dashed up the sideline more than 50 yards before the freshman quarterback knocked him out of bounds.

“Our defense has come out attacking the football better than we have in other springs,” Carroll said.

The offense’s problems, however, were not entirely related to defensive prowess.

The quarterbacks vying to replace Mark Sanchez were handicapped somewhat by the availability of only one center and one regular tight end, and the shuffling of linemen because of injuries. Aaron Corp worked with the first unit, Mustain with the second and Barkley with the third and first units.

None of the three particularly distinguished himself.

Corp did not make a major mistake, but he could not engineer a touchdown.

Mustain was hindered when fullback Adam Goodman and tailback Marc Tyler fumbled after catching passes on successive possessions. Several other Mustain passes were dropped by receivers. “The conditions were not ideal,” Mustain said.

Barkley led two scoring drives, but Thomas’ interception on a first-down play at the two-yard line reverberated as one of Carroll’s teachable moments.

“We give him a chance to make a play and he throws into coverage. . . . That’s just a throw-the-ball-away situation there,” Carroll said.

Said Barkley: “I could have gone a little more outside [with the pass], but I figured why not? But Coach Carroll doesn’t like that, so I won’t do that again.”

Senior Garrett Green, who did not take reps with the quarterbacks the three previous practices, got one series with the second unit and capped a 25-yard drive with a six-yard touchdown pass to David Ausberry. The versatile Green reiterated that he was happy to fill whatever role the coaches require but said he intended to speak to them to “figure out what’s going on.”

Quick hits

Freshman tailback Curtis McNeal impressed, rushing for 48 yards and a touchdown in five carries. . . . Corp completed eight of 13 passes for 70 yards. Mustain was three for eight for 22 yards, Barkley six for 12 for 57 yards with an interception, and Green one for two for six yards and a touchdown. . . . Butch Lewis, who started nine games at tackle the last two seasons, played guard for the first time. Offensive lineman Alex Parsons did not participate because of a broken bone in his hand.

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gary.klein@latimes.com