Ceferin wants the market closed, before the start of the season, in the face of controversial moments that characterize transfers of players across Europe.
English Premier League clubs are expected to vote on the proposal on Thursday, with 14 clubs required to endorse it before it could be ratified.
“I am aware there are serious discussions around Europe regarding the shortening of the summer transfer window and we are following them closely.
“In my view, it is not good when footballers play for one club when the league starts and another club when the transfer window closes. There is a lot of uncertainty for a long time.
“Therefore I would say that the window might be too long and I would support it being shorter,” Ceferin told The Times.