Donadel is expected to link up with Lecce for next term following their relegation to Serie B.
The deal will be mutually beneficial as it will provide the 19-year-old with valuable first team experience and will add impetus to Lecce's bid for a swift return to Serie A.
The midfielder has made just one league appearance, together with a solitary Uefa Cup outing, for the Rossoneri and will be hoping a year away from Giuseppe Meazza will boost his prospects of making the breakthrough on his return.
He had previously been tipped to be included in the deal that saw Sam Dalla Bona join Milan from Chelsea earlier this week.
Milan vice-president Adriano Galliani had initially stated Donadel would move to Stamford Bridge in part-exchange but Chelsea's reluctance to accept anything other than straight cash for Dalla Bona blocked that option.
