The team, and the club, is far, far greater than any individual. Ferguson obviously didn't put the lad on with twenty minutes to go with the intention of subbing him off again - he clearly felt he wasn't playing to his expectations and he'd get what was required from putting Niasse on in his place.
Bigger picture, I thought it was a great move from Dunc. Finally, a marker has been laid to the rest of the side that no individual is bigger than the club, and if you're not playing as prescribed, you're out.
Long overdue, IMO, though I have some empathy for Kean in the sense that there are far more experienced players in this team who have been under-performing for much longer who have had much more lenient treatment.