Since Fellaini was injured for most of the 2nd half of 17/18 its pretty much a given that he was not often used as a plan B so we can look at 16/17. Of course we can't just look at sub appearances because a lot of subs are a few minutes long and are used to waste time and defend the lead rather than a plan B. So lets look at sub appearances that were at least 10 minutes, and see how often they were from a drawing or losing position.
In 16/17 Fellaini came on as a sub for at least 10 minutes 13 times with 6 coming from drawing or losing positions. 1 sub appearance was however a forced sub due to an injury to Pogba so won't be counted as a plan B. So Fellaini came on as a plan B 5 times in 16/17.
Using the same metrics we have the following for 16/17:
Fellaini - 5
Martial - 5
Rashford - 11
Mata - 8
I was looking into the same thing on sunday for the 16/17 season and i have it as Fellaini coming on in 8 games we were losing/drawing granted one of them was the 90' minute which we shouldn't count and the Chelsea game where as you say he came on for an injury.
Burnley 73' score 0-0, finished 0-0
West Ham 85' Score 1-1, finished 1-1
Liverpool 76' score 1-0,finished 1-1
Hull 90' score 2-1,finished 2-1
Bournemouth 70'' score 1-1, finished 1-1
Chelsea 37'' score 0-0, finished 1-0 Pogba injury?
Rostov 48'' score 0-0, finished 1-0
Anderlect 60'' score 1-1, finished 1-0
To be honest i'm not sure a direct comparison can be made between players like Rashford and Fellaini being used as a plan B. When Rashford comes on he will largely play the same role/position as if he started and/or the same role of the player he was replacing, Martial a lot of the times. When Fellaini is brought on in those situations he plays in a different role (target man) than he would if he started in midfield, and the team often changes tactics to hit it direct to him.
Plus with Rashford Mouinho plays him in almost every game, if he doesn't start he comes on as a sub whether we winning, losing or drawing.