Mauricio Pellegrino admitted Southampton never recovered from their nightmare start at St James’ Park.
Pellegrino saw his side fall behind inside 63 seconds to Newcastle, who doubled their lead on the half-hour through the same scorer in the form of on-loan Chelsea winger Kenedy.
Matt Ritchie scored the hosts’ third goal 12 minutes into the second half to compound a day to forget for Saints and their travelling supporters.
“We feel disappointed, because after the first goal the team was really down and we never came back from that situation,” Pellegrino said.
“At this level, you have to be focused from the beginning and we paid the price for that.
“The team was really out of the game. In the second half, we tried with the changes, but after the third goal we never saw the possibility to be in the game.
“You can play bad, you can play in difficult moments or you can make mistakes, but you have to show your character and your spirit.
“The first thing is to react mentally. It is not about football terms because my players know what they have to do, but without the right spirit at this level we cannot compete.”