The 30-year-old Oman international, who signed for the Latics last year following a season-long loan from Bolton, has made two errors in his last two matches.
His blunder allowed Reading to make it 2-2 last weekend before Jordi Gomez's late winner spared his blushes, and he then failed to collect Gareth Barry's shot before Mario Balotelli fired Manchester City ahead in midweek.
However, Martinez said: "I don't think it was an error against Manchester City. Ali in my eyes is one of the best in this league.
"Sometimes you expect Ali to save the unstoppable and that's very unfair.
"The shot from Gareth Barry was moving away a little bit and he came up with a world class save in the rebound but the unfortunate point is that the ball bounced straight into the striker's path and not the defender's path.
"I don't think that's a mistake at all. That's a little bit of misfortune.
"This new ball is quite tough for goalkeepers. His reaction for the first save was outstanding."
Martinez is confident Al Habsi will be fully focused again for Monday's trip to Newcastle.
"Ali is such a confident player and internally he is a very strong human being. I would say his mental strength is his biggest quality," said Martinez.
"As a goalkeeper you need to be able to move on from action to action and he is not affected by that. It's not a confidence issue at all."