"The goal really pleased me, it is very important and was a special strike," said Diego after his crucial goal with just six minutes remaining. "Nevertheless, we cannot be content with 1-1, it was always going to be hard in Milan and the draw makes things harder still, but we are determined to progress in the tournament."
Many of the 36,000-strong crowd had come to see AC Milan's England star David Beckham, on loan from LA Galaxy, while the Italian giants wrestle with the United States club for the services of the global superstar.
But Beckham spent the vast majority of the game on the bench and it was Bremen's Brazilian who kept his side in the competition with an 84th-minute goal after he finally found the net after the home side squandered a huge amount of chances.
"We played the second-half determined to score a goal against AC Milan, that gives me hope," said Bremen coach Thomas Schaaf. "Unfortunately, we do not have superiority in the scoreline. "1-1 against Milan is not bad, but not good enough for the amount of energy we spent. However, if we repeat that second-half in Milan, we would be in with a chance."
Bremen are struggling at tenth in the Bundesliga, while AC Milan are third in Serie A, but it was the Germans who took the game to the Italians.
Diego's super strike mean Bremen go to Milan's San Siro stadium on February 26 for the second-leg with a slim chance, but with 23 shots on goal - compared to Milan's seven - Bremen should have won the game.
The Italians took the lead when Filippo Inzaghi's initial header from a Mathieu Flamini cross was blocked, but the striker was on hand to put Milan ahead by pouncing left-footed on the rebound after just 32 minutes. It was veteran Inzaghi's 66th strike in UEFA club competition.
It looked like Milan had secured a precious away win until Diego rifled the ball into the back of the net after Portuguese striker Hugo Almeida won the ball in the air and the Brazilian made no mistake on 84 minutes.
With Galaxy and Milan tussling for Beckham, the ex-England captain played no more than a cameo role when he took over from Brazil's Ronaldinho after 89 minutes. It remains to be seen if he will be a confirmed Galaxy or Milan player by the time of the second leg next Thursday.