This is basically a collection of links as to why a) this is unsurprising, and b) why it probably isn’t McDermott’s fault.
I don’t think Reading will have come into the season expecting to have a great stab at survival. They may have won the Championship but they ranked a mere 16th in the league in terms of TSR, and had an outrageous amount of good fortune along the way (link).
It’s pretty much impossible to guess how a team promoted to the Premiership will perform once they get there (link), but each of the models I use predicted them to be relegated (link). This was compounded further by the fact they spent almost no money this season (link), and McDermott really didn’t stand much of a chance.
Unsurprisingly they had terrible underlying numbers and were essentially relegated after only 15 games (link). Things didn’t look any prettier after 26 and, despite them getting solid amount of luck (link) they’re languishing at the bottom of the table.
It’s likely too late for Reading, they’re simply not good enough. The best way to improve their chances of staying up would have been a huge investment in the playing squad over the summer. Due to their prudence they’ll probably be relegated in a healthier position than any other club that goes down.
Whether they’ll be good enough to come straight back up remains to be seen – as mentioned at the start of the post they weren’t a particularly good Championship team last time around – but at least they’ll have stability.