While a hamstring injury can be very serious and often recurring, from the sound of it, people aren’t too concerned:

From Rivals:

Chauncey Washington’s “dinged up” leg seemed precautionary. He was still on the field standing and watching the scrimmage, which insinuates that it couldn’t have been that bad.

From Coach Carroll:

“I wish it wouldn’t happen, but the nature of it is pretty mild so we’re not really concerned yet. We’ll see how long it lasts.”

Monday practice report (PM) [USCFootball.com]
Washington Hurts Hamstring [LA Times]