Cobb was scratched from Friday's scheduled start against the Marlins due to back tightness, Evan Webeck of The San Jose Mercury News reports.
Cobb showed improvement during Sunday's start against the Reds and allowed two runs while striking out eight in six innings, but he'll be unable to take the mound Friday. He's considered day-to-day, but it's not yet clear when he'll be able to make his next start. John Brebbia will start Friday's matchup as the Giants utilize a bullpen game against Miami.