Barca needed the returning Lionel Messi to spare their blushes and earn a point after an early defensive mishap allowed Robinho to score the opener at San Siro.
Javier Mascherano and Gerard Pique were at fault in the 10th minute as Robinho played a simple one-two with Kaka before the former tucked home.
But in the 23rd minute Messi, who has recently been sidelined through injury, found space behind displaced full-back Kevin Constant and levelled.
While Milan continued to be dangerous in the second half, Andres Iniesta, Xavi and Messi spurned solid chances to turn the game on its head, meaning Barcelona left Italy with only a two-point lead in Group H.
It was a second consecutive draw for Barca following a weekend stalemate at Osasuna - the first league game in which the Catalans have failed to score in two years - but Martino rejected criticism that his team are running out of ideas.
He told the club's official website: "It's a good point but we deserved more. We only lacked intensity in the first 15 minutes and our punishment was too severe.
"Our play hasn't gotten worse and I don't think we've regressed."
Indeed, the Argentinian coach praised his team for the pressing and direct style of play that allowed his countryman Messi to reach a position from which he could comfortably score against a defensive-minded Milan team.
"Against teams that close themselves off at the back, it's easier to score when you recover possession rather than doing short passes which gives your opponent time to reorganise," he added.
"If we improve collectively, individual performances will also improve."
Midfielder Xavi, whose low shot was snaffled by Milan goalkeeper Marco Amelia, was similarly frustrated.
He said: "It's a shame because we created the chances to win the game.
"(San Siro) is a difficult stadium to play in. We wanted the three points but it wasn't meant to be.
"With the exception of the first 15 minutes, when we conceded a goal, we dominated and we created chances to score."
Next up for the reigning Primera Division champions and current leaders is an 'El Clasico' derby meeting with Real Madrid at Camp Nou and Martino is confident his side can open a six-point gap on their fierce rivals.
He added: "I have no doubts on the team's ability to perform. I would have liked to defeat Osasuna and Milan, but we're doing well and we're still growing."