Rubén Sellés called for more robustness from his team after Southampton were unable to build on their half-time lead at St. James’ Park.
Both sides created chances in an open first half before Saints struck the opener shortly before the interval, as Stuart Armstrong finished off an enterprising counter-attacking move.
But hosts Newcastle responded after the break, levelling the scores within nine minutes through half-time substitute Callum Wilson.
With Saints unable to stem the tide, Eddie Howe’s side struck two quick goals either side of the 80th minute to wrap up the points and leave Saints wondering what might have been.
“In the first half we were very competitive, we knew what to do in every moment, but in the second half we were too passive in terms of defensive work,” Sellés said.
“When you become too passive against a team like Newcastle, you’re going to suffer.
“We had a strategy for the second half to control the jumps and try to play a little bit quick, create situations two against two in their half, but we didn’t connect. I think we can do it better.
“I think they’re a good team, but we showed against Arsenal, against Manchester City, that we can compete against anybody.
“We need to be more robust. We came here, we showed what we are, but we need to put the performance together for 90 to 95, 96 minutes, and that’s why we didn’t make it today.
“We just need to continue, try to put those performances in for longer in the game, and if one incident or one goal is against us, to still stand there and try to get the very best out of the game.
“I think in the second half we didn’t start exactly as we needed, and then after the first incident we were a little bit low. The goals didn’t come until late in the game, but we didn’t show what we can do in the second half.”