Victory against the Piacenza minnows and defeat for Inter Milan by Perugia on Saturday would have secured the Rossoneri a second place in Serie A.
However, the home team were not prepared to give Milan that pleasure as Piacenza aimed to re-conquer the hearts of their fans on Saturday.
The relegation reality has finally kicked in with last weekend's 3-2 loss to Parma which condemned them to Serie B next season.
Gigi Cagni's outfit were leading 2-0 in the first half but threw it all away in the last five minutes.
Rivals AC Milan had the Champions League final to consider and Carlo Ancelotti`s team selection was an opportunity to see the future Rossoneri stars.
The AC Milan starting line-up was made up of five second team players, reserves and AC Milan's third-choice goalkeeper Valerio Fiori.
It was the inexperience of several Rossoneri players, especially in the attacking line, that allowed Piacenza to record an easy win.
The home team were determined to control the match and got off to a good start when they took the lead in the sixth minute of the first-half.
A cross by Piacenza midfielder David Baiocco from the right hand side fell to striker Dario Hubner who was in the centre of the area and whose powerful shot ended in the right side of the net having defeated Fiori.
Ten minutes later, Piacenza extended their lead when defender Mirco Stefani pulled Hubner`s shirt inside the Rossoneri area, to hand Piacenza a penalty.
Midfielder Enzo Maresca did not disappoint and his strong strike left Fiori no chance of saving the shot.
The Rossoneri`s attempts to comeback after a shocking start, proved an uphill battle as Piacenza had most of the possession during the first-half.
However, AC Milan were granted an opportunity to shorten the distance when Piacenza defender Vittorio Tosto brought down Franco-Senegalese midfielder Ibrahim Ba inside the area.
The penalty kick taken by midfielder Cristian Brocchi on the 30th minute of the first-half, beat Piacenza goalkeeper Matteo Guardalben.
The home team could have increased their lead when defender Sergei Gurenko`s cross was superbly followed by a header from Hubner that just missed the AC Milan's cross-bar.
However, Piacenza hit back one minute later when Baiocco`s pass found an unmarked midfielder Marco Marchionni inside the AC Milan area and he headed the ball pass Fiori towards the right of the net to make it 3-1 for Piacenza.
Things did not get any better for the Rossoneri as Piacenza continued the attack after the re-start.
However, Piacenza`s fourth goal did not come until eight minutes prior to the end of the match.
It was wonderful play by Hubner who recovered the ball, went towards Fiori and defeated the goalkeeper with a right-footed shot to score his second of the day.
He finished the season on 14 goals - last season his tally of 24 made him joint top Serie A scorer with David Trezeguet of Juventus.
Once again it was Brocchi that reduced the embarrassing result in injury time, as his shot finished in the back of the Piacenza net to make it 4-2.
Ancelotti`s team finish third, four points behind Inter who drew against Perugia and 11 points behind Juventus.