The 27-year-old completed his move from Newcastle to west London on Friday and took just 35 minutes to open his account for the Blues. Rafael Benitez's side had fallen behind to a fine Jay Rodriguez goal but Ba quickly drew his new side level, getting the final touch on a goalbound Juan Mata effort.
Victor Moses' strike in first-half stoppage time gave Chelsea the lead, with Branislav Ivanovic heading the visitors further ahead seven minutes after the restart. Ba put the tie out of any doubt in the 61st minute, linking up impressively with his new team-mates to slot home past Artur Boruc before Frank Lampard struck a late penalty.
After a slow start, which saw both sides waiting patiently for an opener, Southampton took the early initiative. Jason Puncheon sent a fine through ball to Rodriguez, who took a touch before expertly slotting past Ross Turnbull with the outside of his right foot.
The 22nd-minute strike jolted the visitors into life and Eden Hazard soon came close with a curled effort that went narrowly wide of the far post.
Ba's link-up play was improving as the half wore on and Saints were having to defend resolutely. However, their resistance only lasted for 35 minutes as the new boy opened his Chelsea account. Hazard played through Mata, whose dink over Boruc was creeping in only for Ba to fire into the roof of the net.
Chelsea were playing with new-found confidence and took the lead in first-half stoppage time, with Moses collecting a cutback from Ashley Cole to power home a low left-footed strike.
The Blues continued in the ascendancy after the restart, with Mata seeing a strike deflected wide by Guly do Prado shortly after the restart. The Spaniard was involved yet again in the 52nd minute, sending over a left wing cross that Ivanovic headed home at the near post.
Nine minutes later Ba capped off a fine move he started, latching onto a pass from Hazard to slot home under pressure from Southampton captain Jose Fonte. The Portuguese defender looked to be badly injured in the challenge and was stretchered off, with Jos Hooiveld coming on in his place.
Lampard added further gloss to the scoresheet from the spot seven minutes from time after Steven Davis was adjudged to have handled the ball. The 34-year-old sent Boruc the wrong way to equal Bobby Tambling's record of 25 FA Cup goals for Chelsea.
- Sports & Recreation
- Artur Boruc
- Branislav Ivanovic
- Victor Moses