DeGale, 32, faces his British rival in a non-title contest at London's O2 Arena on 23 February.
Eubank Jr, 29, won in September after being beaten by George Groves in February, while DeGale vacated the IBF world super-middleweight title in July.
"After he has lost to me I am not sure where he is going to go," said DeGale.
"Call this a retirement fight. Whoever loses can knock it on the head. Whoever loses, game over.
"A lot of people think I am in decline but they are in for a shock."
At a London news conference, DeGale referred to his rival as "limited" and said Eubank Jr was "well down" the list when ranking the calibre of opponents he has faced.
"James talks retirement but it has never even come into my mind," said Eubank Jr. "In February it will be a painful lesson. I have been training to fight you for years. I know everything about you."
The contest will be a pay-per-view broadcast on ITV Box Office and is the first of a new deal involving US promoters Premier Boxing Champions after the organisation struck a deal with the broadcaster.
Olympic gold medallist DeGale lost his world title in December 2017 but won it back in an April rematch with Caleb Truax, only to give up the belt as he wished to focus on chasing "massive fights".
He insists Eubank Jr is "too arrogant" to listen to his new coach following a 2018 in which he lost to Groves and scored a routine win over JJ McDonagh in September.
"I have a new team around me," said Eubank Jr. "I haven't really had an official trainer.
"Ronnie Davies has been by my side but hasn't trained me, he's been more overseeing things. I have trained myself.
"For this it will be different. I have been working with a guy called Nate Vasquez from Las Vegas. He has been living with me. It is good to have a solid guy there who is with me 24-7, working with me on specifics rather than me going through the motions."
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