To answer your rudely phrased question, last season (2018-19) in the most recent 10 games that were "in the balance" (which for these purposes I decided would be a 12 point final margin or less, ymmv), Javin averaged 23.3 mpg. He hit 20 minutes or more in 8 of the 10 games. His average fouls per 40 minutes in those 10 games was 5.49, which is high but certainly wouldn't preclude playing 20 mpg.
In other words, once again your eye test assertions are way off base.
My recollection is that most people were expecting him to be a role player, coming in.
You may believe guys ranked #35 should be better than role players at Duke, but in the entire RSCI era (not counting current players), we've only had seven (non-transfer) players ranked lower than 30 who played more than nominal roles on the team:
Quinn Cook (#31)
Josh Hairston (#32)
Matt Jones (#34)
Marshall Plumlee (#61)
Dave McClure (#71)
Lee Melchionni (#121)
Tyler Thornton (#144)
Of these, I would describe all except Quinn Cook to be "role players."
So, I would describe your expectations as unrealistic.