Ciro Capuano gave the visitors the lead with a cracking long range shot midway through the first half and the Sicilians could easily have doubled their advantage by the break.
Instead 37-year-old striker Filippo Inzaghi, playing because of injuries to Brazilians Alexandre Pato and Robinho, levelled the scores on the stroke of halftime after Ronaldinho's splendid assist.
"It's always difficult to play against so many defenders," Ronaldinho told Sky TV.
"We've wasted an opportunity and it's two points lost but if we carry on like this we'll soon succeed even if at the moment things aren't going our way."
Milan, third last term and without a scudetto since 2004, beat Lecce 4-0 in their season opener under new boss Massimiliano Allegri but have since stuttered.
They lost 2-0 at promoted Cesena last weekend before struggling to beat Auxerre 2-0 at the San Siro in their first Champions League group game on Wednesday.
New forward Zlatan Ibrahimovic, who scored both goals against the French outfit despite not playing well, had another inconsistent display on Saturday but midfielder Kevin Prince Boateng again excelled.
Serial strugglers Catania have four points from three matches like Milan.
Earlier Fiorentina's dreadful start to the season dragged on with a 2-1 home defeat to Lazio which left Sinisa Mihajlovic's men with just one point from their three games.
Adem Ljajic put the hosts ahead from the penalty spot on 19 minutes before Cristian Ledesma struck back just after the half hour and Libor Kozak netted the second-half winner when Sebastien Frey could only parry a Hernanes shot.
The Florence club, who reached the knockout stages of the Champions League last term, lost coach Cesare Prandelli to Italy in the close season and failed to significantly strengthen in the transfer window.
Lazio, relegation candidates last term and beloved former club of Fiorentina coach Mihajlovic, have six points.
Champions Inter Milan visit Palermo without the injured Wesley Sneijder in the pick of Sunday's main programme of games.
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