Ideally, Nehra shouldn't have been in the team.
If Nehra was indeed in the team, He should have bowled his quota/taken off for being too expensive before the 47th over and Zak should've got the very last over and Dhoni should've ensured Harby got his full 10 overs before.
If Dhoni had a tact-fail and finished Zak's 10 before the 50th over, He should've just given it to Harby.
The idea behind a fast bowler being given is the last over is that he puts the balls fast - in the blockhole. Nehra has not bowled an accurate full delivery more than three times in the 21st century so you don't really have an argument about choosing Nehra over Harbhajan. Another over-looked fail is Harbhajan being given only 9 overs overall. When you have two decent bowlers, You make sure that both of them bowl their full 10 overs each. This incident clearly speaks of no planning and cloudy thinking - leaving your best bowler of the day one over for the 50th over and then being an instinctive ****wit and giving the ball to Nehra. Total joke tbh, No defense at all.