This isn't new with Luka. Each time the Mavs lose, Luka just finds himself on the injury report again as questionable and not only that, but Doncic himself references it often. He did a lot in the OKC series, which was probably Dallas' toughest series in the West and went 6 games.
Guarantee you IF Boston goes up 2-0 tonight, you'll hear Doncic spend a lot of time diagnosing his injuries and explaining how much of a grind it is, etc.