the only philosophical question I can think of regarding Garrard is 'what is the sound of no hands clapping?'
I flat dropped him a few days ago after picking up eli as a throw-in on another trade.
To answer your question, no you shouldn't cancel or undo the trade. Should you feel bad? That's up to you. But as the guy who has accepted a trade after lineups were locked (dumb, I know) for the weekend and then seen my new guy that I haven't even taken delivery on go down for the season, thems the breaks. I wouldn't feel bad about a trade he accepted in good faith, and he should bear you no ill will.
And garrard was probably inconsequential, he used the trade to get Grant and add to his RB depth.