Des has been in over 4 months - much longer than pre-season. Browne and Edwards are the only players to have been injured the majority of that time, and they're still not playing so largely irrelevant.
No one would've been too upset if Des' first couple of games weren't great, but the issue is it started not great and hasn't got much better. You're right that the performances under Manning weren't always brilliant/fun to watch, but we largely still won - isn't that the sign of a good manager? Manning brought in players and had them playing really well. Leigh & Rodrigues spring to mind - both were great under Manning. Yes Leigh had an injury, but he's been back 2 months now and is nowhere near as good as he was. Rodrigues.... well what's happened there then???
Point is, Des has had ample time to work with the squad, make his own signings, implement his style of play etc. It's very telling that after over 4 months we still don't actually know what his style of play is.