Ole Gunnar Solskjaer had revealed he planned to get rid of Darmian, 29, before the end of the European transfer window.
Now, according to reports in Italy, the clubs have agreed to a permanent deal, with United taking an £11.5m loss on the full-back.
United brought in Darmian from Torino in 2015 for £12.9m - and an initial one-year loan deal had been on the table.
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