I can only assume(hope) that Mancini is confident of securing another DM before the deadline closes, Rodwell at this time is no where near an adequate replacement for De Jong.
De Jong is class and would be a much weaker team without him, people think he has no vision but when watching him play for Holland he seems to take more responsibility and have seen him thread some quality passes, we already have players for that so De Jongs role here was to pass it to them.
Not sure who could replace him, probably a 1% chance of DDR coming.