The result leaves Milan's top four chances in the balance ahead of the final game of the season.
“There's no point thinking about what might've been. We have to focus on the next match," Pioli told Sky Italia.
“We weren't at the level of our recent performances, in terms of tempo and quality. We had the game of a lifetime against Juventus, then with Torino, so it's another of those with Atalanta. We go again.
“In tonight's game, we felt a bit of everything. We knew a win would finish it, so there was some tension, some fear, some misplaced confidence that we could win by playing ugly, but we have to fight for every point.
“We should've moved the ball quicker and created more advantageous situations, that's just it."
He added: "We've done great things this season, there is disappointment this evening and we know Atalanta are the only side we've been unable to beat during my tenure at Milan.
"We evidently have to bring out our best in order to achieve it."