Clemson offered no details about the violation when the school announced the NCAA's decision Friday, a few hours before the Tigers opened their season against Western Carolina.
Skara, who coach Brad Brownell has said is the team's best defender, won't play for Clemson until Dec. 16, against Florida.
Clemson referred questions about Skara's suspension to the NCAA. NCAA spokeswoman Stacey Osburn said in an email that the organization had no further information to share about Skara.
Skara averaged 6.5 points and 3.1 rebounds as a sophomore at Valparaiso two years ago. Valparaiso athletic spokesman Aaron Leavitt said in an email to the AP that the school would have no comment on the matter.
Brownell said he is disappointed with the NCAA's decision.