Swansea gifted Northampton both goals in a 2-0 defeat as the home side slumped in the season opener.
Swansea's Paul Connor challenges goalkeeper Lee Harper
Early damage was done in the third minute, when Swansea full-back Stuart Jones' backpass was intercepted.
Former Swans man Marc Richards could only toe-poke against the post, but Scott McGleish followed up to plant the loose ball in the back of net.
Swansea had chances to level, but eight minutes from time Sam Ricketts headed a cross into his own net.
Swansea: Gueret, Jones, O'Leary, Tate, Ricketts, Robinson, Monk, Britton, Martinez (Maylett 72), Forbes (Oli 62), Connor.
Subs Not Used: Nugent, Fisken, Murphy.
Booked: Connor, Robinson, Gueret.
Northampton: Harper, Chambers, Willmott, Murray,
Low (Bojic 76), Rowson, Smith (Youngs 85), Cozic, Galbraith,
Richards (Sabin 69), McGleish.
Subs Not Used: Bunn, Jaszczun.
Booked: Galbraith, Willmott.
Goals: McGleish 3, Ricketts 82 og.
Referee: M Fletcher (Worcestershire).