Nielsen had a good game in the 3-3 with Celtic and then was dropped for 3 crunch games. Dropped for a keeper we had no intention of offering a new contract to. He clearly wasn't trusted by the previous regime (or maybe McManus chose the team?).
As it happens, I think Twardzik has the potential to develop into a keeper that will play at a higher level. 22/23 is young for a keeper. For what he lacks physically, I think he has a wee bit of steel about him and plenty of natural abity. Unfortunately he has a defence in front of him that will undermine any potential to develop at present.