Manager Nathan Jones pointed out deficiences in both boxes as the reason Brentford ran out comfortable victors in the capital.
Two goals in three minutes just before half time put the Bees in command, before Mathias Jensen distinguished any hope of late drama with a header 10 minutes from time.
Falling to successive Premier League defeats, the Saints boss delivered an honest assessment at Gtech Community Stadium.
“Really poor,” he said. “At spells in the first half we weren’t aggressive, didn’t play quick enough and didn’t play with any tempo, and when we did we looked the better side, but defending our box we have to be better.
“They ran harder than us, outrun us, outworked us, and then when they put the ball in the box they looked more threatening than us.
“We put more balls in the box than them, we had plenty of opportunities to put the ball in, but every time we put the ball in they were touch tight, they marked, they did the basics really, really well and we were a mile off today.
Deadline day signings Paul Onuachu and Kamaldeen Sulemana were handed their debuts off the bench at half time, with Jones encouraged by their individual cameos.
“They made a difference,” he said. “They gave us an impetus and we looked better, put more balls in the box and were a better team.
“Late on, they only put one ball in the box, people go to sleep, don’t follow a runner, don’t compete and get a free header from six, seven yards. We didn’t have any free headers from six seven yards. You have to defend your box better. It happened before I came, they didn’t defend the box well, and we’ve tried to improve that but it didn’t show today.”