He has a SF-PF body but he matchs with PG's or SG's on defence. So those matchups are mismatched.
Just compare Luka's size with his opponent team SF's, PF's on this playoffs.
Luka: 6-7, 230lb
This round:
Draymond: 6-6, 230lb
Otto Porter Jr.: 6-8, 198lb
Andrew Wiggins: 6-7, 197lb
Previous round:
Jae Crowder: 6-6, 235lb
Mikal Bridges: 6-6, 209lb
Cameron Johnson: 6-8, 210lb
First round:
Royce O'Neale: 6-4, 226lb
Bojan Bogdanović: 6-7, 226lb
Danuel House Jr.: 6-6, 220lb