Ravel Morrison has returned to parent club West Ham from Cardiff City on compassionate leave.
Brighton boss Sami Hyypia is weighing up a move for former France captain Alou Diarra.
Martino said in a press conference on Tuesday night that he was sure fellow Argentinian Messi and his father Jorge had put in a good word for him with the Barcelona hierarchy before the Catalan giants offered him the job.
Both Messi and Martino are from Rosario and Messi has in the past spoken in glowing terms of his compatriot.
However, the four-time Ballon d'Or winner has said he had no influence in Barca's decision.
"I have nothing to do with Martino's signing nor do I have to give any explanations. It was down to president (Sandro) Rosell and the club. I said before that he seemed a good coach, but I don't have to explain anything," he was quoted as saying by AS following Barca's friendly defeat to Bayern Munich.
"I don't know him personally, I've never met him. Not him nor Luis Enrique, the other one they've been talking about.
"I think he's a good signing, he showed at Newell's and with Paraguay what he's capable of. Hopefully he'll be good for us."
Martino, who led Newell's Old Boys to the Argentinian Clausura title last season and guided Paraguay to the quarter-finals of the 2010 World Cup and final of the 2011 Copa America, has agreed a two-year contract with Barca and will be presented tomorrow lunchtime.
The 50-year-old replaces Tito Vilanova, who stepped down from the post last Friday as he continues his battle with cancer.