The famed MSN forward line contributed 122 goals last season as Barca claimed a Champions League, La Liga and Copa del Rey treble in a goal-laden campaign.
The threesome have also played a key role this term as Barca clinched the FIFA Club World Cup title in Japan, while also maintaining their Liga challenge and easing through to the knockout stages of the Champions League.
Aleix Vidal and Arda Turan have been added to the squad following the ending of the club's transfer ban but Luis Enrique is still on the look-out.
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Speaking at a news conference ahead of Wednesday's Copa del Rey return against city rivals Espanyol - Barca lead 4-1 after the Camp Nou opener - he was asked which area he thought needed strengthening. He replied: "I don't know how to answer but it's a very good question.
"Right now, probably in attack, if we were to bring someone in - that's probably the key area. But it would be nice to add more to all areas."
The former Real Madrid and Barca midfielder conceded, however, that January was not always the best time to get the right players in.
He added: "It's harder in winter, nobody wants to lose important players.
"It's not an easy market but we do want to improve our squad and we're looking at opportunities."
Messi claimed the fifth FIFA Ballon d'Or of his career in Zurich on Monday night with Neymar coming in third behind Cristiano Ronaldo.