i have a couple of questions / comments:
1) what are the players' stamina? i think this plays a big role in substitution decisions.
2) i have similar problems (i like to have a stronger, rather than weaker, bench). i have never had a situation where my coach let a sub play 24 minutes, partly because of stamina, i think. so the way i read your projection that your new guy comes off the bench getting 24 minutes suggests to me that his talent level is substantially higher than your homegrown player. if this is indeed the case, then i, too, would say play your best players, period.
3) it is possible that coming off the bench hot shot may be going against the bench guard for the opposing team, increasing his effectiveness. but this is borderline over-coaching -- or is it?
4) neither player apparently gets into foul trouble with any regularity?