Messi's season has been disrupted by a couple of minor injuries and the Argentine said on Chinese social networking site Weibo at the weekend he was still not fully fit.
At home to Espanyol on Friday he failed to score for a fourth successive La Liga match in which he played the full 90 minutes, his most barren run in almost six years.
"I think that this must be the third news conference in the last five days and more or less the 20th question about Leo's situation," Martino told reporters ahead of Wednesday's Champions League match at home to AC Milan.
"And I will continue saying exactly the same thing," added the 50-year-old, who took over from the ailing Tito Vilanova in the close season.
"What I am is happy to have him, to be able to coach him and have him in the team.
"Let's wait for the next match and see whether he continues to contribute what he has been contributing to Barcelona, when everything he does is decisive in the match."
Martino said Messi had not been given a special recovery programme and was training normally with his team mates.
"It could be that Leo is not feeling fine because he has had two injuries in a short period of time," Martino said.
"Obviously his form may have dipped as a result and perhaps he doesn't feel at 100 percent.
"He is training in the way he needs to depending on the number of games we have, nothing specific."
Messi scored the equaliser in last month's 1-1 Champions League draw at Milan, his fourth goal of the competition in two outings this season following a hat-trick against Ajax Amsterdam in September.
His record in La Liga is also one most players would be delighted with - eight goals in 10 appearances - and Martino said his compatriot was a victim of having set the bar so high for himself.
Victory for Barca at the Nou Camp on Wednesday would put them on 10 points with two matches left and send the 2009 and 2011 winners through to the last 16.
Milan have five points from three games, with Celtic on three and Ajax on one before their match in Amsterdam.
(Writing by Iain Rogers in Madrid, editing by Ed Osmond)
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