By FOX SPORTS staff
The Rossoneri could not come up with a reply to a first half brace from Lionel Messi, before David Villa extended Barcelona's advantage further with his goal 10 minutes into the second period. Jordi Alba then added a fourth in front of an exultant Camp Nou, completing the Catalan side's progress into the last eight of the Champions League.
Allegri told uefa.com that Barcelona's pressing game exhausted his players and they caved under the pressure.
"In the first half-hour Barcelona played extraordinarily well. They were very good especially in pressing us high, making it very difficult for us to come out from our own half with the ball. They scored three of their four goals after we lost possession with bad mistakes," he pointed out.
"Football is also a matter of centimetres because we hit the post when we could have made it 1-1 and a few moments later they scored the second with a ball going between the legs of our defender."
Although their La Liga rivals were early favourites to qualify for the next stage of the competition, Allegri admitted that Milan had harboured dreams of pulling off an upset.
"We are sad because we had great expectations after the first leg. Football is like that; in the first leg we were lucky when we scored our first goal, tonight [Tuesday] we were unlucky when we hit the post at a crucial moment of the game," he shared.
"Barcelona have great players like Messi and Iniesta who can dribble past any defenders easily, even after running for 90 minutes. But my players never gave up, even after Barcelona's third goal."
AC Milan are currently third in the Serie A and will look to retain that position when they welcome Palermo to the San Siro on Sunday.