The Saints may have failed to reach the heights of Sunday’s win at Old Trafford, but Ronald Koeman’s side never looked in any real danger of losing.
Ipswich’s quick start had a packed-out Portman Road dreaming of an upset, with Stephen Hunt finding the net early on. However, the veteran Town midfielder’s effort was chalked off for offside and Mick McCarthy’s promotion chasers managed just one other shot on Fraser Forster’s goal.
Saints were pragmatic rather than pretty, though, with the decisive goal coming after 19 minutes when Long reacted quickly to produce a lovely finish after James Ward-Prowse was tackled.
The much-changed visitors failed to create many more clear-cut chances but were able to extend their unbeaten run in all competitions to seven games, setting up a fourth-round clash with former manager Alan Pardew and his Crystal Palace side.
The only negative on the night was the hamstring injury sustained by Victor Wanyama — a particular concern following the pre-match news that fellow defensive midfielder Morgan Schneiderlin faces up to three weeks out with an adductor strain.
The France international was one of five alterations to the win at United and perhaps those changes were behind the Premier League side’s slow start – one which Ipswich thought they had capitalised on.
With less than five minutes on the clock, Hunt raced onto a Kevin Bru through ball and beat Forster, only for the offside flag to bring the Irishman’s celebrations to a halt.
That let-off gave Saints’ defence a jolt but those in front of them struggled to find any coherence, with a wayward Wanyama effort from distance all they could muster until the 19th minute.
Neat team play and a clever turn saw Ward-Prowse through on goal. Ipswich captain Luke Chambers produced a wonderful challenge to deny the England Under-21s midfielder, only for the ball to fall for Long to fire home an impressive first-time strike.
Wanyama and Ward-Prowse had efforts of differing quality from distance as they attempted to add to the opener, while Long twice bore down on former Southampton goalkeeper Bartosz Bialkowski.
Dusan Tadic saw a goalbound shot blocked as the first half entered stoppage time – a period in which the visitors key man Wanyama sustained a game-ending hamstring injury.
Things could have got even worse for Saints after their former striker David McGoldrick got free, but the Irish international could only fire into the side-netting.
Long continued to prove a nuisance when play resumed after half-time, when Town at last began to make an attacking imprint.
Substitute Tyrone Mings, provided a fine cross which Darren Ambrose directed goalwards while a dangerous cross from the bandaged Daryl Murphy called Jose Fonte into action
As the clock wound down Bru failed to find the target, as did substitute Graziano Pelle at the other end, and Saints progressed.
IPSWICH: Bialkowski, Chambers, Smith, Berra, Parr, Ambrose (Stewart 82), Hyam, Bru, Stephen Hunt (Mings 55), McGoldrick (Bajner 63), Murphy.
SOUTHAMPTON: Forster, Gardos, Fonte, Bertrand, Clyne, Ward-Prowse, Wanyama (Steven Davis 46), Reed, Targett, Long, Tadic (Pelle 67).
Ref: Graham Scott.