The Argentina ace has failed to score in his last four Liga matches, drawing a blank in Friday's 1-0 derby win over Espanyol, and only managed to bag one goal throughout October.
Despite concerns over his recent displays, Martino is convinced his compatriot will enjoy a "great day" against Milan on Wednesday and has called on Blaugrana faithful to continue to trust in his abilities.
"Everything that Messi does is decisive," Martino stressed at a press conference. "I'm very happy to have him on the team.
"If we go back, there has always been a point in the season when Leo has not scored the goals that he usually does and yet we are always coming back to the same story.
"He has gone a few games without scoring but has contributed to the team, as always. The goals will come.
"The only thing we can do to stop all this negativity is to keep working and keep trusting in one another.
"The most important thing is trust. Messi will have many more great days and I bet tomorrow will be one of them."
While the Rossoneri are genuinely struggling in Serie A, the Blaugrana are riding high at the top of La Liga, having dropped just two points after 11 rounds of action.
However, the Catalans have still copping flak for a number of allegedly sub-standard showings of late, prompting Martino to bite back on the eve of his side's meeting with Milan at Camp Nou.
"We are also in crisis, so there is not much difference in relation to how both Barcelona and AC Milan are coming into this game," he said, smirking. "In this way, we are pretty evenly matched ...
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"But we are always the first to say that we have much room for improvement."
Barcelona go into their Group H showdown with Milan knowing that a win will guarantee them a place in the last 16.