Milan are clinging on to fourth place, the Italian top-flight's final Champions League qualification spot, having looked certain to return to Europe's elite club competition following a run of five straight wins prior to their sudden slump.
"Today was the first time that we suffered physically. We struggled during the second half, therefore, we gave them space," Gattuso told Milan TV.
"We didn't have the strength to get back for their goal. We could've won but we also could've lost. The strength that we've had for three or four months was missing.
"We gave off the feeling of fragility. We must remain calm. We know that we're going through a tough spell and we must get out of it.
"There are still eight matches left. We know how many points are still on offer and how important they can be. We must recover mentally and physically. We've lost some certainties at this moment in time.
"Many people like to talk about tactics but the difference is made through desire and physicality. You could see this today. It's not a question of our formation but rather how we play, how we can hurt opponents. The team tried to do so but you could see that we were lacking that cutting edge.
"We were waiting for an opportunity. When you win five matches but then face some difficulties, the ball starts to weigh heavy. Things that happen that should be prevented and this is the moment we're in. We weren't precise, we struggled and at the end these performances can happen."
Gianluigi Donnarumma and Lucas Paqueta were both substituted due to injury in the first half, adding to Milan's fitness problems after Franck Kessie was unable to play.
"I'm concerned because I saw the footage and it was a bad sprain," Gattuso said of Paqueta's injury. "The ankle swelled up straight away. He'll have a scan and we'll see what the doctors say.
"We must also get Kessie back fit, who yesterday dropped out of training after 10 minutes. He tried today but he had to stop. He's had a scan and there is a bit of inflammation.
"Suso hasn't trained for two days after the knock he picked up to his foot in Genoa. We must try to get these injured players back fit."