The Italians were forced to hold a firesale of their top players to try and balance the books at the end of last season, with striker Zlatan Ibrahimovic and defender Thiago Silva among those to leave.
Alexandre Pato has also left since Milan were eliminated by the same opponents in the quarter-finals just under a year ago.
Mario Balotelli, their only marquee signing this season, is cup-tied against Barca, leaving the seven-times European champions a pale shadow of their former selves.
Milan seem to have struggled to come to terms with their new reality - a reflection of the difficulties facing Serie A where clubs can no longer compete with the financial clout of other top European leagues.
At times Milan have been happy to play the role of underdogs, with president Silvio Berlusconi and coach Massimiliano Allegri emphasising they are now a young team building for a financially sustainable future.
But there was none of that talk on their website in the build up to the match as the club preferred to hark back to their past record.
"A great European city and a fabulous stadium are ready to host the game of the year," it said proudly, casting a veil over Milan's current situation.
"Certain stadiums bring out a certain atmosphere especially when it comes to European fixtures. Milan-Barcelona is the match between (one of) the most successful clubs in the world and currently the best club in the world."
POOR MILAN RUN
Things have certainly changed since Milan outclassed Barcelona 4-0 in the 1994 final.
The Italians have won only one of 10 meetings since then, a 1-0 win at San Siro in 2004, and none of the last seven.
The teams have twice met in the knockout stages with Barcelona winning 3-1 on aggregate in last season's quarter-finals and 1-0 on aggregate in the 2005/06 semi-final on their way to winning the title.
Without Balotelli, who has scored four goals in three games since his move from Manchester City, Milan's hopes of reversing that sequence would appear to be minimal despite an impressive domestic run which has seem them climb to third in Serie A.
Four-times World Player of the Year Messi looks unstoppable for Barcelona.
Midfielder Xavi is back in the Barcelona squad after missing the last two games but David Villa will not make the trip as he is still recovering from kidney stones.
Striker Pedro churned out the usual platitudes when he faced the Spanish media on Monday.
"Milan has always been difficult," he said. "They have titles and a great history. It will be difficult, as it always is. They are always dangerous."
AC Milan: 32-Christian Abbiati; 20-Ignazio Abate, 17-Mario Yepes, 5-Philippe Mexes, 21-Kevin Constant; 8-Antonio Nocerino, 10-Kevin-Prince Boateng, 18-Riccardo Montolivo, 4-Sulley Muntari; 11-Giampaolo Pazzini, 92-Stephan El Shaaraway.
Barcelona: 1-Victor Valdes; 2-Dani Alves, 3-Gerard Pique, 5-Carles Puyol, 18-Jordi Alba; 6-Xavi, 16-Sergio Busquets, 8-Andres Iniesta; 4-Cesc Fabregas, 10-Lionel Messi, 17-Pedro.