Athletic trainer Mike Goforth told reporters in Blacksburg, Va., on Monday night that Exum went for a follow-up appointment with Dr. James Andrews and was not given clearance to play.
Andrews, one of the top orthopedic surgeons in the country, performed Exum's knee surgery.
Exum will have another follow-up exam in one and a half to two months, meaning he could miss all of September as well. The second-team All-ACC selection a year ago tore his ACL, lateral and medial meniscus, and fractured a bone in his knee during a pickup basketball game in January.
Exum and coach Frank Beamer had held out hope that Exum could be healthy enough to play against the Crimson Tide on Aug. 31. But rehab from this injury generally takes eight to 12 months.
"It's just one of those things that takes time," Beamer said Monday. "It just needs to be right for him and for us."
With Exum out, Virginia Tech will have to rely on some young players against one of the best offenses in the country.