Ibrahimovic, who won a Serie A title with Milan in 2011, has left LA Galaxy following two years in MLS.
- When does the transfer window reopen?
- All major completed transfer deals
He has been heavily linked with a return to his former club, but sources have told ESPN he must lower his wage demands in order to facilitate a move.
"The longer it goes on, the harder it becomes," Boban told Italy's Radio Uno. "He is 38, has been inactive for two months.
"It's not easy to start again. Especially considering there are only five months until the end of the season. I can't repeat what Paolo [Maldini] said.
"Let's hope Ibra makes the right choice -- even for himself. We need to see how the next matches go. Ibra is a different solution to the others, he can give a lot in the short-term and for this reason he is unique.
"Let's see if there is the absolute necessity to act now or at the end of the season. Let's see if, aside from this, we need to make attacking signings. But Ibra is unique, a different player in comparison to the others even if he isn't young.
"He has an extreme character. When we started the conversation started, however, we were in a different league position."
Milan are unbeaten in four league matches since a spirited 1-0 defeat to Juventus and have risen to 10th in the Serie A table.
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