Johnson’s injury is more serious

USC tailback Stafon Johnson is not expected to play against Arizona, and the former Dorsey High star said Thursday that his foot injury is more serious than first thought.

Johnson, a sophomore, said what was first diagnosed as a left foot bruise suffered during USC’s victory over Washington State on Oct. 22 was subsequently revealed to also include a sprain. The injuries have prevented Johnson from practicing for two weeks.

“Once we got the bruise out we could tell what it really was,” said Johnson, who leads the Trojans with 378 yards rushing and is averaging 8.2 yards a carry.

Johnson said he changed insoles and took other measures in an attempt to get back to full speed, but he took part in only a few drills Thursday.

“I can play if it came down to it but it’s not up to me,” he said.

Senior Chauncey Washington will start against Arizona. Coach Pete Carroll has said that senior Hershel Dennis would be the main backup, with senior Desmond Reed, sophomore Allen Bradford and freshman Joe McKnight also available.

Jeff Byers worked again at center as Matt Spanos rested the cut fingers on his right hand.

Spanos and Carroll said the senior would play against the Wildcats.

“He’s got a handful of stitches but when they tighten it all down and tape it up and protect him, he’s going to play,” Carroll said.

Spanos has not practiced this week but he said he would not have trouble snapping the ball to Mark Sanchez, who will be starting at quarterback for the first time.

Carroll said senior Thomas Williams and junior Clay Matthews would play strong-side linebacker against Arizona. . . . USC has two players on its roster from Arizona: defensive lineman Everson Griffen and center Kris O’Dowd. Arizona has 42 Californians.

gary.klein@latimes.com