The former Reading and Bolton midfielder, 29, was signed on a short-term deal until January.
Karacan will provide cover in the middle of the park with Ben Thompson joining Portsmouth on a season-long loan.
He has been a free agent since being released by Wanderers at the start of the summer.
“It will take a few weeks for Jem to get up to speed and we’re looking to maybe include him in the cup game next week,” said Millwall manager Neil Harris.
“Then we’ll assess him after that for the first-team and league duties.
“There is a possibility we might see him before the international break. If not, we have to be patient and wait until after.
“Jem is a super-fit lad but we still don’t want to take any unnecessary risks at this stage of the season.”
Thompson played the full 90 minutes for Pompey as they thrashed Oxford United 4-1 on Saturday.
Harris has already stated the 22-year-old’s long-term future lies at The Den.
“The first thing we do after the final whistle on our game is check how Ben, Tom King, Sid Nelson and James Brown have done [at their respective loan clubs],” said the Lions chief.
“They are all really important players for us moving forward.
“We want them to play as many minutes as they can to play in our first-team in the future.”