This is definitely unfortunate. I like Smart and what he brings to the table, but he's the most replaceable player on the team.
There's not any real drop off between his play and that of Jaylen Brown or Terry Rozier in terms of total basketball talent.
Now his defense can be exceptional, but the Pacers have a bunch of guards that are barely NBA players. They're old, unathletic, and completely unintimidating without Oladipo. The last thing the Celtics need is a "lock-down" defensive guard in this series.
I'm actually glad Smart won't be available for this series because I think Brad bends over backwards to provide Smart with unnecessary minutes for a role player to the detriment of other players on the team.