The 31-year-old made the comments as he revealed he will be out "for a while" to undergo right shoulder surgery.
Degale is the IBF super-middleweight champion, while fellow Briton Groves holds the WBA super-middleweight belt.
"Groves has gone backwards," said DeGale. "I'm not being disrespectful but he's not got the same sharpness."
Beijing 2008 Olympic gold medallist DeGale added: "He has, however, fulfilled his dream of becoming world champion and it makes our potential rematch even bigger."
"Negotiations will be difficult and the terms will have to be right for the fight to be made."
Groves, who beat DeGale six years ago to win the British super-middleweight title in a majority points decision, won a world title at the fourth attempt by stopping Russian Fedor Chudinov last month to claim the WBA belt.
DeGale's last outing saw a unification fight between him and Sweden's Badou Jack end in a controversial majority draw.
DeGale said a fight scheduled for 1 July had been agreed with Callum Smith but that he "decided to go down the WBC route and will challenge for the vacant world title later this year".
On his injury, he added: "My arm will be in a sling for a few days and I'll be given exercises to do on my own.
"A few weeks later, I'll start my rehab with my physio and within eight to 10 weeks I should be able to spar. I will definitely be able to fight before the end of the year."
George Groves "thinks he is bigger than he is" and does not deserve "a 50/50 split" from a rematch, says the man he beat in 2011, James DeGale.