Itachi has the worst stamina in the akatsuki while Ay has bijuu chakra level, do the maths
His stamina is actually quite good. Now are you referring to his chakra reserves or his capability to continue to to fight? If you're referring to the second option you're terribly wrong.
Even with a injured hip near his leg due to sasukes windmill shuriken[
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] and being near blind and sick he was still capable of evading sasukes katon[
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][
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]. Even after coughing up blood and unaware at the moment[
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]. He still reacted to sasukes dragon fire katon[
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]. And once again even after falling to the floor and getting back up in time he was still capable of evading the dragon katon[
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]. So as you can clearly see his stamina if your referring to him being able to still fight under extreme circumstances is great.