We shouldn't assume he needs *BIG* minutes to come over here. He's never actually gotten consistent big minutes in Europe - that's not how they like to play.
For what it's worth, Roku's stats this year with his regular team suggest that he's only a 12-20mpg type of player. Doing a bit of rounding, but this season he's averaging 17+mpg, 7.3ppg, 2.8apg, 1.5rpg. In general he shoots a pretty high percentage (.553 FG%, .472 3pt% albeit on only 1.2 attempts/game).
He was really impressive in FIBA play last summer (25mpg, 12ppg, 5.3apg, 4.3rpg .571 3pt% over 3 games) and you might be able to get that type of production at the NBA level.
We have no idea what his defense will be like, but he seems to have a high motor and I think he can get on Thibs' good graces based on making things happen on broken plays.