The result eventually saw Russell Martin sacked by Saints on Sunday night.
Armstrong said at the final whistle: “I can’t put it into words. It’s one of the worst first halves I’ve been involved in, just couldn’t get going.
“We said to get at them, got the press wrong at the start and they just cut through us. Quality players do that in this league, and the goals were unacceptable.
“It kills you, it kills you in any sport. If you go down straightaway it’s a tough place to come back, especially in the Premier League.
“It’s a tough league, and when you’re playing against teams like this and you get punished it’s a tough one. I don’t know what to say. The early goals killed us.
“We go in at half time and it’s just ‘no more goals’. It’s just about getting everyone together and regrouping. It’s a tough one, because everyone’s feeling the same pain.
“It’s not easy to get the boys going after being four, five nil down. We had to come back out fighting, showed our character, kept running as hard as we can. It’s one of those nights.”