Setting up a tantalising continental clash with city rivals Inter, the Rossoneri edged out Napoli to progress, but they are still struggling to secure a top-four finish domestically; Lecce, meanwhile, last tasted victory in February.
Match preview
A second close contest with Serie A's supreme leaders in the space of a week may have seen Milan held to a 1-1 draw, but that result at Stadio Maradona was still enough to see off Napoli and reach the Champions League's final four.
Veteran striker Olivier Giroud atoned for an early missed penalty by opening the scoring down in Naples, and despite conceding a late equaliser, Milan were able to eliminate Italy's top team of recent months and their nailed-on successors as champions.
Last Saturday, though, a much-changed XI continued the Rossoneri's below-par run of results in the league, with a spectacular 4-0 win at Napoli being their only domestic success since February.
After falling behind within a minute against Bologna, Tommaso Pobega's strike then salvaged a point which kept Milan fourth in the table and just ahead of Inter; however, the subsequent reversal of a points deduction for Juventus sees them sit fifth with just eight games left to play.
A fairly dismal defence of their Scudetto has left them 22 points adrift of top spot, and Stefano Pioli will now seek a response to his team's run of just six points from as many league games.
Hosting Lecce, then, perhaps comes at an apposite time, as Milan's record against the southern side is particularly dominant: they have lost only two of the clubs' 33 top-flight meetings and were last beaten by them some 17 years ago.
It is necessary to trawl even further back through the history books to find Lecce's one and only Serie A win over Milan at San Siro - that sole success dates back to October 1997.
However, they did manage to take a two-goal lead in January's reverse fixture at Stadio Via del Mare, before being pegged back to 2-2 by the final whistle. As a result, the Salentini can go unbeaten in both seasonal meetings with the second-city giants for the first time this century.
Lecce drew against Sampdoria last time out, ending a run of six consecutive league defeats, but remain involved in a battle against relegation straight back to Serie B as they are only five points clear of the drop zone ahead of the run-in.
Away from home, the Giallorossi may have lost their last three, but they have also picked up more points on the road this term than in Puglia; and having kept the fewest clean sheets of any Serie A side this season - the most recent came against Cremonese in February - it is clear where Marco Baroni's main problems lie ahead of the trip to Milan.
For that reason, the prospect of keeping Rafael Leao and co at bay for 90 minutes may have been giving Lecce's defence several sleepless nights.
AC Milan Serie A form:
L
D
L
W
D
D
AC Milan form (all competitions):
L
W
D
W
D
D
Lecce Serie A form:
L
L
L
L
L
D
Team News
After resting all 10 outfield players that started the first leg against Napoli last time out in Serie A, Milan may make less dramatic changes to their lineup on Sunday evening.
Stefano Pioli will again be without Zlatan Ibrahimovic due to injury, while captain Davide Calabria misses out as he serves a suspension, but the Rossoneri coach otherwise has a full squad to choose from.
Alessandro Florenzi could replace Calabria at right-back, while Pierre Kalulu is likely to come in for Simon Kjaer at the heart of the hosts' defence; promoting either Ante Rebic or Divock Origi from the bench would offer Olivier Giroud a rare rest up front.
Lecce, meanwhile, could even name the same XI which started against Sampdoria last week, as defensive duo Marin Pongracic (ankle) and Kastriot Dermaku (thigh) are once again the Giallorossi's only men absent through injury.
After his opening goal versus Samp, Assan Ceesay should again lead the line in a three-man attack, with Milan loanee Lorenzo Colombo perhaps consigned to a second-half cameo.
AC Milan possible starting lineup:
Maignan; Florenzi, Kalulu, Tomori, Hernandez; Tonali, Pobega; Saelemaekers, Diaz, Leao; Rebic
Lecce possible starting lineup:
Falcone; Gendrey, Baschirotto, Umtiti, Gallo; Oudin, Hjulmand, Blin; Strefezza, Colombo, Di Francesco