Because of Milwaukee's team size and a high-level front-line of Giannis/Brook/Portis that all have some level of inside-outside skill, Rob Williams is going to be so important in that Bucks series. He was clearly still getting his rhythm and conditioning back against the Nets, but we're going to need regular season Rob to get by the Bucks.
While I'm sure Jayson, Jaylen, and Smart will get switched onto Giannis at times, I don't think we can rely on the Jays to be primary defenders on Giannis the way they were on Durant because Giannis draws so many fouls with his physicality. That means Horford, Grant, and Rob are going to need to be the primary defenders of Giannis, and they're going to need all of their fouls and energy to make life hard on him.
It will be interesting to see who Giannis guards. From past precedent, it seems like Milwaukee likes to keep him in more of help role and away from the other team's star. If they do that, I would assume Giannis guards Horford, Jrue take Tatum, and Brown should have awesome matchups to attack (much like in the Brooklyn series). However, if Giannis can guard Tatum credibly (as Tatum just did to Durant), that would allow Jrue to check Jaylen and leave us with less favorable matchups to attack. I will be very interested to see how Milwaukee matches up to us defensively.