Sharingan can copy seals of a move it sees. This is how Itachi would copy FTG.
We also saw a 2-t and 1-t sasuke copy lees taijutsu speed. I am sure a 3-t/MS pair of eyes could copy minatos reflexes and speed.
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At this point, itachi can use all of minatos own kunai teleportation seals against him, at minatos speed. This is a big weakness for minato as he carries a bunch of these
on his person at all times. itachi can then teleport directly to minato and 1-shot him with a kunai or whatever. Itachi after this fight would be even more powerful as he could now use FTG on demand.
Other Simple ways Itachi could defeat Minato
- Tsukuyomi (minato has no knowledge, and has been seen looking into the eyes of his opponent every fight)
- Finger genjutsu - this techinque doesn't even require the victim to look at the finger, it just instantly works
- Koto, if itachi in his prime