The Nerazzurri got off to a flying start through an early strike from Ivan Perisic before they doubled their lead courtesy of a headed finish from Stefan De Vrij. However, they were undone by goals from Andrea Belotti and Soualiho Meite, leaving them without a win after two matches.
Spalletti had no explanation as to what exactly went wrong for his side, after it all seemed to be going so smoothly in the first half.
"It's not easy to explain why we lost our way in the second half, because we seemed surprised by a long ball from midfield," Spalletti told Sky Sport Italia in the post-match press conference (via ESPN).
"We didn't give the right consideration to the ball and there can be no other explanation. We then didn't have the right reaction to pressure. It's difficult to explain how we conceded those goals, but clearly the reality is that the team is struggling with pressure."
Many considered Inter to be one of the strongest challengers to reigning Serie A champions Juventus this season, but Spalletti insists that there is still a lot of work to do before they can be considered true contenders.
"It's all fine and good, you telling me Inter are the anti-Juve, but you have to be the anti-Napoli, Roma and Lazio first. Because then you say it and let it all rest on my shoulders.
"If you ask me about that performance against Sassuolo, I tell you we're not the anti-anyone, but that doesn't mean I don't have the faith in the potential of my squad. The media do their job, but it does create pressure."
Inter will hope to change their fortunes as quickly as possible, as they travel away to face Bologna on Saturday. Bologna and Inter are level in the table, after Bologna drew 0-0 against Frosinone on Sunday.