ROME, May 18 (Reuters) - AC Milan will be without Kevin Prince-Boateng for their trip to relegated Siena on Sunday (1845 GMT) after the midfielder was ruled out with a knee injury on Saturday. Milan were boosted by the return of Riccardo Montolivo, who is back in the squad published on Milan's official website (www.acmilan.com) after two weeks out with a hamstring injury. Massimiliano Allegri's side are third in Serie A on 69 points and will guarantee themselves a place in the preliminary rounds of the Champions League with a win. ... More...