Simple put Nelson is on the way out, his form has been declining somewhat over the last few years, as Bristol pointed out he has made one or two memorable and costly mistakes; I reckon he has probably got another season in him at league level.
So we need a long term replacement, and first in line is obviously Morris. Considering he hasn't played at this level before to be fair to the lad we need to give him more than the odd substitute or cup game to bed into the team, so now we are safe, assuming we can keep a similar level of performance, we can afford to give him a run of five or six game to see if he is up to the task.
Unlike Steve, however, I don't think there is a massive gap in class between the two based on the limited appearances Morris has had. The both have their strengths, Nelson seems to be a better shot stopper, while Morris seems to control the 18 yard box better. Regardless to what has happen previously I don't think Morris deserves to be dropped based on his performances