The Argentina star has denied influencing the appointment of the former Newell's Old Boys boss as Tito Vilanova's replacement
The 44-year-old was confirmed as Tito Vilanova's replacement - who stepped down to continue his recovery from a cancer relapse - on Tuesday, after a successful stint in charge of Messi's childhood side Newell's Old Boys.
That has led to rumours the four-time Ballon d'Or winner had a strong influence in the club's latest appointment, something he was quick to dismiss after Barca were beaten 2-0 by Bayern in a pre-season friendly on Wednesday.
He said: "I don't have anything to do with the signing of Martino and I have nothing to explain. It's Rosell and the club's thing. At that time, I said that he looked like a good coach, but I have nothing to explain.
"I don't know him personally and have never seen him or Luis Enrique, who was also linked. He seems to be a good signing as he has deomontrated with Newell's and Paraguay that he is capable. Hopefully he'll do well here."
Martino also enjoyed success with Paraguay, guiding the nation to the quarter-finals of the 2010 World Cup - where they were beaten by Spain - and the final of the 2011 Copa America, only to be beaten 3-0 by Uruguay at the final hurdle.