The Nesmith pick makes sense for the team the Celts wants to be, i.e. with Kemba healthy.
The Celts need a wing with legit size to defend 3s and smaller 4s who can also reliably hit an outside shot. Grant Williams is not really a wing and isn't a reliable shooter yet. Semi is too small and just isn't much of a threat offensively in general.
Unfortunately right now without Kemba the team seriously lacks players who can handle, pass, and shoot. So it's very hard to find minutes for a guy who, at least right now, is basically a shooting specialist.
Meanwhile you've got Pritchard playing in lineups with Jeff Teague and sometimes Marcus Smart. Why? Because the team needs the ballhandling he provides to keep the offense operating. That's not going to work defensively in the playoffs. I'm nearly certain we will not see Teague-Pritchard lineups in the playoffs. If we do, that's a problem.