Unless Nate's shooting improves a good bit, he is not really a "spark" type player. His shooting performance has generally been better the more minutes he plays. A lot of guys are like that, so that's not a knock. But it means he's not a great option if you need an offensive spark.
Then again, until we see more of this team, we don't know how much Nate has improved or whether guys like Kenny or Brandon or even Stilman might be good "sparks."
One guy who almost certainly will be a spark off the bench is Seventh. From what we saw in the exhibition game, the only thing holding him back as a designated spark is a suspect 3-pt shot.
Theo is probably our best spark off the bench, but from what Roy said, he was slated to be a starter.
When Theo comes back, will he have enough time left to become a starter or will someone else have settled into that role? We don't know.
The longer Theo's out, the more likely he'll end up being used as a spark off the bench - because he already knows that role. He doesn't yet know the starter role to that same degree and we may not want him (and his teammates) to have to learn that role if it's late in the season, even if he's 100% healthy. For those who prefer Theo as a SF and not a SG, that could be a blessing in disguise, since he will presumably take over at SF next year.
Bottom line: the SG starting role belongs to Nate for the moment, but he'll need to play better than he did against Pembroke or else Kenny, 7th or Brandon may take it away from him.