The league made the announcement on Wednesday.
Alexander suspended May 2 for four games for violating the substance abuse policy.
The Panthers had hoped Alexander's return this week would provide some help with Pro Bowler Greg Hardy out indefinitely while dealing with a domestic abuse case.
The NFL says Alexander's suspension begins immediately. He will be eligible to return to the active roster on Dec. 15.
Alexander, a fourth-round draft pick in 2012, played in 12 games and had 15 tackles and one sack last season. He played in 16 games a rookie with 2.5 sacks.
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