Sarkic made 33 appearances for the club last season after joining from Wolves in August 2023 in what was a great season for the keeper.
The stopper would have turned 27 today and his teammate George Honeyman has said that he did not think Millwall's players would ever fully get over the loss of their friend.
"It's a weird one in football because people come and go every off-season, so in a weird sense we're kind of still just waiting for him to walk through the door," the 29-year-old said.
"And, you know, give the lads a big hug, see how your off-season's been, all the normal sort of craic."
"Obviously, I think we're looking forward to celebrating him with the fans on the first game back at The Den," Honeyman added.
"It's a work in progress, and I don't think many of us have had many experiences of this sort, so that's where we need each other and the staff.
"It's tough. The lads are up and down with it, but we're a pretty tight-nit group, so we'll take comfort in that we've got each other."
The club is planning a mural painting of Sarkic at the Den while a "Matija Sarkic Award" will be launched in 2024-25 which allows fans to vote for their top save of the season.