Part of the problem is that sports radio (and even more knowledgeable fans) focus too heavily on the fact that the Celtics were "one game away from the Finals" last year despite no Kyrie. While correct, context matters:
Brad did a masterful job replacing Kyrie's minutes and contributions. Rozier stepped up and replaced some of Kyrie's ball handling responsibilities, and developed a nice shooting touch to go with it. Brown and Tatum were able to replace some of Kyrie's scoring, which was really an unexpected gift. Horford stepped up as well. But the reality is that the Celtics won only one road game in 8 tries, and that win was in OT; during those 8 games, they scored 92, 102, 86, 101, 92, 86, 102, and 99 points. That 95 ppg average was a whole 7 points less than their regular season average on the road. And they lost Game 7 because they could only put 79 points on the board.
What they couldn't replace was Kyrie's consistency, which matters a lot in a long and deep playoff series. This season it's a similar story. They can replace Kyrie with Rozier over small stretches. But of the 8 games that Terry has started, only one was against a quality opponent (Utah), and that was a big L where Rozier was a -18 despite scoring 22 points.