You dont play a player for 30 mins against fresh opposition and use one of you subs inside the first half. Your play him in the last 30 mins when the that opposition is weaker and tired. Any idiot can deduce that i am sure.
Maybe not for thirty minutes, but he could play the first half or maybe the first hour. Depends which way you look at it, I'd rather see us go out firing in the first half, put teams to bed early, have the match comfortably won by the hour mark and then rest our best players as an where necessary.
If you look a games this season, we tend to do better when Duncan starts than when he comes off the bench (see below). Whilst it is true the opposition will be tired in the last 30 minutes, so will the other Alty players. As a rule of thumb, games start of quite tight as everyone is fresh and working hard, by having more position in the first half the opposition will tire quicker as they spend most of the time chasing the ball.
Bringing on Duncan when we are losing may sound like a good idea as he certainly had the ability to create a goalscoring opportunity, but seeing as his team mates are fatigued, he has to work harder to cover for them - this also leaves us vulnerable to counter attacks and conceding late goals.
I seem to remember United doing something similar and resting all their best players, then bringing them with half an hour to go when they were a goal down. They spent most of the time chasing the ball rather than effectively using it, something that would have been possible if they started.
anyway: stats with and without Duncan in our starting lineup this season:
When Duncan starts:
15 points from 9 games. (1.67 ppg)
When Duncan comes off the bench:
4 points from 5 games. (0.80 ppg)