Villa did not muster a shot on target in a dismal display and stayed just a point above the bottom three. Long's eighth-minute strike and Armstrong's stoppage-time breakaway, after Villa goalkeeper Pepe Reina had gone up for a corner, eased Southampton -- not so long ago appearing relegation-bound itself -- to relative safety in the standings.
Villa could well be back in the relegation zone after the weekend. Southampton kicked off with the worst home record in the league, but it did not show as the hosts tore into a Villa side with woeful form on the road.
The opener came when Moussa Djenepo reached the byline and cut the ball back for Long to bundle in at the near post. Long can hardly be described as a prolific goalscorer, but this was a seventh goal in his last seven appearances against Villa.
Danny Ings, by contrast, has been lethal in front of goal but when he went through one-on-one with Reina, he put his shot too close to the Spaniard. A couple of lengthy injury delays briefly affected Southampton, and Villa had the ball in the net shortly before halftime — only for Mbwana Samatta to be flagged offside before he converted Jack Grealish's pass.
After the break, Villa continued to be panic-stricken by any set-piece into its area. Kortney Hause sliced one corner over his own crossbar and Southampton defender Jack Stephens poked Ryan Bertrand's free kick wide.
Reina saved a fierce volley from Pierre-Emile Hojbjerg before taking one for the team when he was booked for shoving Long outside the area following a poor attempt at a header. The closest Villa came to forcing an equaliser was substitute Trezeguet's effort which fizzed across goal and wide.
Its misery was compounded in stoppage time when Armstrong rolled the ball into an empty net from 35 yards after Reina had long since given up the chase.
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