I am sure there have been worse than this, but pre-2006-07 Ashes I remember it being announced that Fletcher had lost the plot and done the following:
Jones in for Read- Jones was a poor wicket keeper who could on occasion look good with the bat. Read was a far superior keeper who didn't look pretty with the bat but had improved vastly with it and done everything asked of him.
Giles for Panesar- The Aussies regarded Giles as a joke. Monty eventually came in and took 8 wickets at Perth. Giles retired with an average over 40.
Anderson for Mahmood- Doesn't look bad now, but at the time Jimmy hadn't played any cricket for months and was hammered by the Aussie batsman.