Stephan El Shaarawy netted a crucial equaliser for the visitors.
"We struggled a little at the start, losing the ball in banal fashion and allowing them easy chances," Allegri told Sky Sport Italia.
"In the second half we controlled the ball better and above all defended better, as in the first half we were too wide between the sections and that allowed passes through.
"It's a shame, as we had so many chances to seal the win even after the equaliser, so now it's all open ahead of the second leg.
"If we had won and especially with a two-goal cushion then we would be looking at this match differently.
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