Remo Freuler and Ruben Vargas' goals earned the Swiss a first win over the Azzurri since 1993 as the Euro 2020 winners were completely outplayed.
"We need players with more physicality, when you think you can play with your quality... but you lose intensity, it's only natural that you need to mix things up," Spalletti told reporters.
"What happened tonight did not come down to one single cause... if you don't do more in tempo and intensity, it's hard to even compete on an even footing.
"We were under par in terms of intensity, tonight I made six changes (to the line-up) because I wanted to allow the players to be fresh, but the tempo was the same."
Despite saying his squad lacked pace, Spalletti took responsibility for the defeat.
"There's no blame on anyone, I want to stress this," said the 65-year-old.
"The responsibility is mine, I'm always on the side of the players, I thank them for having brought me here (to the Euros)."
The coach thought it "only natural" for questions to be asked about his future at the helm.
However he defended himself by citing his relative inexperience in the job after taking over in August 2023, following the surprise resignation of Roberto Mancini.
"All of the other coaches had 20 games in charge, some had 30 before the Euros, I only had 10," he said.
"The responsibility is mine, I picked the players. Of course, this is part of a process where I need to get to know the players."
Italy beat Albania in their Group B opener but did not lay a glove on Spain in a 1-0 defeat and then scraped through with a 98th minute equaliser against Croatia in a 1-1 draw.
Despite shuffling his pack against Switzerland, nothing Spalletti tried came up trumps.
"Having tried out a number of things over the course of this experience, I do come away with the notion that I have to change things, I'm convinced of that, I have to change things now," continued the coach.
"(That said) it's not as scandalous a result as you're trying to paint it to be... today we were under par, and against Spain, but not against Croatia, we made it through a tough group.
"However we didn't see a team that's defined, in terms of fundamentals, the foundations we can actually build upon."
