كتب : Omar AbdelAziz
Boro conceded two goals in the opening six minutes after the interval, before West Ham's Dean Ashton completed the rout with a third on the hour mark.
"We started okay, we weathered the pressure and at the end of the first half we looked like the stronger team," Southgate told the club's official site.
"But we had an awful, awful start to the second half and that spell has ultimately cost us the game.
"I don’t think we were switched on enough, we haven’t really made a challenge from the kick off to the ball ending up in the back of our net."
The Boro boss promised before the Hammers game that his side is capable of scoring goals, but after the embarrassing result, Southgate lamented missing the opener in the first half.
"That’s the end scoreline and that’s what we've got to go home with. The first goal in any game is crucial and we had chances to get that in the first half," he added.
Egypt striker Ahmed Hossam 'Mido' played full 90 minutes for Boro, but the 24-year-old failed to find the Hammers' net throughout the game.