I don't really know how you can argue that this proves he can't be trusted.
He hasn't been given a chance all season and only got a game today because Rio got injured and Valencia had already been taken off.
He's still very young so some brashness is to be expected. He came on today and wanted to be involved in everything. It might not have been the best thing to be doing but he was at least showing some commitment and effort, and in all honesty the only thing he was guilty of with the foul was over-committing.
It was a stupid thing to do, I don't think anyone would deny that, but footballers do stupid things from time to time, even ones that start every game. It's hardly a shock that a young player desperate to prove himself did something daft while trying to make an impression.
We've persisted with Evra at left back despite him obviously having ambitions of being the new (old) Gareth Bale, Buttner seems to have been his cover by default despite looking completely inept more often than not, Rafael and Evans have even played there ahead of him despite not being left backs.
He's watched a winger and two center backs play at right back ahead of him, despite the former not being up to scratch and the latter two looking uncomfortable at best.
He's seen the likes of Young, Kagawa, Hernandez, Fellaini and Cleverley get regular games despite barely ever having an impact or doing anything of any use. All the while he's either been left out of the squad entirely, or included in it with absolutely no hope of getting on the pitch short of a defensive crisis.
It's easy to say that he's been unimpressive for the last 2 and half years when he's spent half a year without getting a kick, a year shafted out on a shitty loan deal at a doomed club where he was constantly played out of position, and essentially a year out injured.
You can't completely disregard the fact that before this he was regarded one of the most talented young players we had in the squad, so much so that the greatest manager of all time picked him to play a CL final against Barca.
I really hope we stick with him and give him a proper chance to prove himself because it'd be ridiculous and grossly unfair to base your entire opinion of him on his showing today.