It's possible Duke's "struggles" are due to a lack of pre-season work (summer, practices, exhibition games). But when it comes to missing regular season games, I don't buy it. For example, for the first three games, every year we have the same number of "tune up games." This season, however, we've performed decidedly worse in those games than in recent previous years.
Below is a table of the last 10 seasons. The first column is our "unadjusted" average margin of victory in our first three games (adjusted only for home/away/neutral). The second column is our average "adjusted" margin, using Pomeroy's end-of-season ratings (latest ratings in the case of 2021). For both unadjusted and adjusted, I assumed a 4.5 point home advantage in previous seasons and a 1.5 home advantage this season (0 point advantage at neutral sites). Each sample includes two home games against "lesser" non-conference competition, and one game on a neutral court* against a "top shelf" opponent.
* (except this season against Michigan State, which was at home)
Code:
Year unadj adj
2015 42.00 37.58
2019 30.33 35.38
2020 28.00 35.27
2017 25.00 27.93
2018 21.00 27.52
2012 14.67 25.07
2014 17.67 23.13
2013 14.67 18.73
2016 14.00 14.37
2021 7.17 5.49
As you can see, this year's team was significantly worse than any of the previous year's teams (in fact, it's significantly worse than any Duke team since Pomeroy's ratings were invented). As I said, some of that difference may be due to a lack of pre-season work, but I expect a lot (most?) of the difference is due to this year's model simply not being as good a team as previous Duke teams.