Of course, Itachi has limits. But they're not exhausted, here.
He's superior to Orochimaru by a good margin, and totally out of Tsunade's reach. Tsunade has two options: Engaging him and trying to distract him with hand-to-hand combat, or staying back and working as a healer.
- The latter would be useless, since Orochimaru is very well capable of undoing damage by himself. Tsunade wouldn't have much to do as a healer in this fight, and therefore would be totally wasted.
- The former, however, would be rather useless due to Itachi's predictional abilities or Susanoo and would quickly end with her being entrapped in Itachi's ordinary Genjutsu or Tsukuyomi. Mental damage is unlikely to be healable by Byakugou. In a worst case scenario Itachi will use Amaterasu on her.
That being said, Tsunade isn't much of a threat, here. Orochimaru is neither, as I assume Edo Tensei is restricted in this fight. Amaterasu will prove ineffective on him, thanks to his rebirth-technique, but Tsukuyomi should still show effect in case of eye-contact. Manda and his various huge summonings are incredibly strong, but badly matched against Itachi, considering he can easily control them with his hypnotic Sharingan. Even if he can only control one at a time; taking control of the strongest and making it fight the others or even Orochimaru will mostly render them useless. In case he's pressured by them, Amaterasu will save the day.
That being said, the rest Orochimaru has shown will be blocked by Susanoo or countered with Itachi's extraordinary speed and Bunshin feints. The real danger is Orochimaru's persistance, as it might exhaust Itachi and has a possibility to push him towards his limits. But if he's combining feints with clever use of Genjutsu, which he definitely will, he'll sooner or later create an opening to seal Orochimaru.