Even if everyone is healthy, I think between small injuries and Stevens giving guys days off for rest, you'll see around 30 player games being missed by Horford, Morris, Hayward, Tatum, Brown and Smart. Those minutes for those player games will go to Semi and he could easily average 15-20 minutes per game for those 30 games. And that's with good health. Add that to small regular rotation minutes and games with big minutes due to blowouts and I just can't see Semi averaging less than 10-12 mpg for as many games as he plays.
On the whole this looks logical. I thought I'd flesh it out a bit.
Picking four guys out of your list...
Smart has averaged 65 games in four seasons.
Brown 74 in two seasons...
Assuming that they play their recent average number of games, that's already a total of 40 missed games. Let's assume really good luck, and cut that in half, leaving 20. If you multiply that by the total average minutes that each of them played last season (119), you get 2380 minutes missed - for just those four.
I'm suggesting a scenario way more optimistic than your 30 missed by six players, and there is still a ton of available minutes.
Ojeleye played 1150 last season, which was 15.8 per game. (He had 4 DNP-CD's and 4 out for the back injury.)
This is not a prediction, it's to demonstrate how workable getting Semi rotation minutes would be, even given Boston's depth. Obviously we could go through the list of likely rotation players to get a fuller sense of what's predictable for available minutes; it would only add to what's available.
The real issue is whether or not he merits rotation minutes, and it looks to me as though the coaching staff have already settled that for themselves. They had him in at crunch time frequently last year, and he played a big role in the playoffs.
For some players there's an effort gap after the first year; not Ojeleye, who continues to get high marks for his work ethic (from Larranaga in Vegas, for example). He showed evidence in SL of work on his dribble drives and playmaking.