All that for him playing like 4 hours next season ? He is not at the level of United quality player.
If he wants to play some football before retirement send him to lesser team to play last full season and then make a deal that he will come back is not impossible to do that.
If that role you mentioned is the main thing give it to him now, he is not good enough to play anymore at that level.
I do not see how he needs to stay here to be an ambassador or else. I am sure there are still teams that he could contribute and have fun with that.