Milan were beaten for the second time in the league on Sunday as Duvan Zapata and Ricky Alvarez secured a 2-0 win for the home side at the Stadio Luigi Ferraris.
And Montella, who was returning to his former club, was critical of Milan's display, claiming no single player performed to the expected level though he believes with work the season can be saved.
"We didn't like the performance. We were insufficient both in determination and quality," Montella told Mediaset Premium.
"We were still in the game even if Samp had several chances in the first half, but we didn't have mental clarity and we have to figure out why.
"Nobody put in a good performance. We are still a work in progress. We've got a lot of talent in the side, so more work is needed going forward. This is not a project that can be written off already.
"I was disappointed by the performance and we need to work out why it happened. We were psychologically running on empty.
"Perhaps, for some, the third game in seven days was a problem, but we need to analyse the situation."
Montella had selected Cristian Zapata to deal with the threat of Duvan, but the Samp forward pounced on his cousin's poor defensive header to net the opener and he believed it was a deserved win.
"I think we deserved this victory, we fought to the end and must continue in the vein of this performance," Duvan Zapata said. "It was wonderful to score today and play well."