The actor told TV Guide that vampire Klaus "definitely isn't confirmed" for a potential fourth season.
"There's very much a threat he won't make it to season four," said Morgan. "Or even if he does make it... that it won't be in the way you think."
He added: "That's a very real threat - I'm genuinely worried by it."
However, the English star also suggested that Klaus could be written out of the CW drama without being killed off.
"I imagine they [could] do something like [put him for] eternity in a box," he speculated. "They keep him alive, but he's no threat to anyone."
The Vampire Diaries airs on Thursdays at 8/7c on The CW.
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Watch a clip from the next Vampire Diaries below: