Sasuke was having trouble initially tracking Juubito's speed even with EMS. Madara trumps Juubito in speed (due to having half of Kurama within him and Hashirama's Sage Mode compared to the scraps Kin and Gin provided), yet he did not keep track of Guy until he was right in front of him. If Guy can momentarily make it difficult for a Juubi Jin with Sage Mode to keep track of him with his line of sight, then Itachi has no hope of tracking Guy with his sight at all, especially since he does not possess EMS like Sasuke, danger sensing thanks to Sage Mode, or physical reflexes anywhere near a Juubi Jin.
Since he can't keep Guy in his line of sight, he's not landing Amaterasu or Tsukuyomi at all, making Susano'o his only option. As a result, Guy can just blitz Itachi from behind with Hirudora, destroying/heavily damaging Susano'o in the process, then throw another one while Itachi attempts to recover. Or he can play the waiting game whilst in the 6th Gate by dancing around his attacks and bombarding him with Asa Kujaku, seeing how that Gate does not give him anywhere near the strain that the 7th Gate does to maintain.