Vincenzo Iaquinta scored twice for Juve, with Olof Mellberg and Pavel Nedved also getting on the scoresheet.
Spalletti was forced to field an inexperienced line-up due to a raft of injuries and lamented: "I am really annoyed to have lost with such a clear result when we had this series of unfortunate situations that affected our build-up.
"Too many players are in bad shape. I am disappointed by the result and especially sorry for our fans. The lads worked hard and Juventus are an opponent that always brings something extra out of you.
"We must accept the defeat, which was deserved, and get it back into our heads that we have 15 days to train hard and fix these physical problems. We will also work on the tactical issues that went wrong tonight.
"Considering the situation just before this game, we tried to play and wanted to win. It must give us even more fire and determination to reach our objective, which is fourth place."
Simone Loria got a rare start and a potentially crucial confidence booster, as the much-criticised defender scored the temporary equaliser.
"Loria is an exemplary professional who always trains hard and this is the best way to create great performances," added Spalletti.