Just a day after completing his move from Newcastle, the Senegalese striker led the line in the absence of the rested Fernando Torres and got his Chelsea career to an ideal start with a tap in to equalize Jay Rodriguez’s opener for Southampton.
From then, Chelsea always looked likely to secure victory and they duly did so in impressive manner with further strikes from Victor Moses, Branislav Ivanovic, Ba and a Frank Lampard penalty.
Rafael Benitez clearly demonstrated the importance the FA Cup holds to him this season as he fielded a strong lineup with Juan Mata again exceptional and Eden Hazard adding creativity from the left.
Southampton, who also took to the field with a strong side, equipped themselves well in the early stages. Nigel Adkins’ men looked well organized as well as disturbing Chelsea’s rhythm with their pressing. And the home side grabbed a fine opening goal midway through the opening period.
Jason Puncheon produced a precise first-time through ball to find Rodrigiuez who peeled off Gary Cahill and took touch before coolly flicking the ball past the advancing Ross Turnbull.
Coming off the back of a defeat to Queens Park Rangers that may have all-but ended their Premier League title challenge, Chelsea and Benitez would have been under serious pressure have they tumbled out of the FA Cup at St Mary’s.
There was little panic at going behind, though, and gradually they started to find the quality to pierce through Southampton’s defenses.
The visitor’s equalizer came as they worked the ball quickly upfield and Southampton got caught stretched, allowing Hazard to drift into the box and square the ball for Mata. The impressive Spaniard met the pass with a good first time touch past the challenge of Maya Yoshida and dinked an effort that was partially blocked by Artur Boruc, but Ba was on hand racing in to apply the finishing touch ahead of Steven Davis from a yard out.
From the moment they got back on level terms, Chelsea settled into complete control of the matchup and it always looked a matter of time before the dominance would be reflected in the score line. With Southampton looking to get into half-time on level terms, Chelsea duly took the lead deep into injury time.
Moses found Ashley Cole down the left then waited in space for the pull back that found him with precision and the winger took a touch to set himself before drilling a low left foot shot into the corner.
There was no let up from Chelsea after the break and seemingly nothing Southampton could do to stem the inexorable tide in their opponent’s favor.
Adkins, though, would doubtless be disappointed by the simplicity of Chelsea’s third goal seven minutes after the restart. After a corner broke down, Mata got the ball on the left and was allowed time to chip the ball onto the head of Ivanovic who had remained forward and the Serbian defender headed home from six yards.
Chelsea were probing at will and soon added a fourth just past the hour mark. Hazard was given far too much space on the edge of the box just to play a simple ball through to meet the run of Ba who got there just ahead of Jose Fonte to poke the ball low past Boruc.
With the result now beyond doubt at 4-1, both managers, for very different reasons, took the opportunity to make changes ahead of other challenges ahead.
One of Southampton’s substitutes looked sure to at least pull a goal back with 20 minutes remaining, but Ross Turnbull made a fine save at the feet of Steve de Ridder and then recovered to block the follow up from Davis.
The afternoon was almost given a perfect finish as Ba missed out on a hat-trick when Boruc saved well with his leg from the new signing’s far-post header.
There was still time, though, for Frank Lampard to grab yet another goal to take him toward being the club’s all-time leading scorer. Davis was adjudged to have handled the ball on the edge of the box and Lampard drilled the resulting penalty low into the net.