The 27-year old Dutchman has been linked with the Fire for months and has now made the move to MLS from Belgian club Germinal Beerschot.
MacDonald comes to Toyota Park with the Fire having loaned out Uruguayan forward Federico Puppo to homeland club Defensor Sporting.
"I don't know exactly [when I'll be ready] because for me, I just finished holidays," he said.
"So I normally have to have preparations. But I'm going to try to speed it up now and play as soon as possible."