BTW, how far is the idler shaft sticking out?
Picture?
If it is only sticking out about 3/16" of an inch or so, it may be possible to carefully tap it back into place with a plastic hammer. No hard banging. It should tap in easily with the flat cutout on the end aligned where the little retainer stop plate will rest. If it does not. Stop.
If the idler shaft is sticking out more than about 5/16" or so, the O-ring seal on the other end of the shaft may have been lost and there would be no way to just tap it in as the O-ring would bind and not seat correctly. The rear cover would have to come off to reposition everything.