As Nige said, it might be the angle of the photo shot ? ….. but also the centre of the main idler is still nice and square ….. The carrier roller is a very common failure, excavators alike …… The roller is shot ! ….. not even worth checking if the bearing is good or not ….. BUT WHY ??? is the concern ….. I'm with the others here, it got seized and the rail dragged across for a long period of time, but not seeing the whole picture, as in: is it wore the same on the inside as out ??? ….. if not, then would in my books prove to have stopped turning, also if not ??? ... another common occurrence is rocks etc. get jammed up between the top frame and top of the pads causing either the main pedestal to bend, causing misalignment, or the main shaft of the idler got bent. As posted above they are cheap and in your purchase price can be adjusted.
Another 'ol quick trick I was taught years ago, Was Grab, with both hands, either side of a Pad, anywhere there might be a bit of slack is easier, and Rock it back and forth, Left to Right as hard as you can !!! ….. If it seems to be excessive movement, then suspect the pins within the track rails to be worn out ….. by the looks of the pic showing the drive sprockets they are in excellent shape, Nice and square tips of the teeth, the gap or spacing where the pins contact are Not elongated, looks nice and uniformly round ... pending on total hrs of the machine, I would either suspect if it was a higher hour machine that those drive sprockets were replaced not long ago …… If you found the Movement as I mentioned before to be excessive and those sprockets to look like new then the rails have the appearance to be stretched or as the pins to be worn bad, will cause the track pitch to change and now wear out your drive sprockets really fast !!! ….
If I get a chance later I will post some pics of what worn sprockets off of my Excavator look like …. they are worn out and I have to do a total undercarriage overhaul, Rails, Drive Sprockets, bottom rollers, and top rollers, front idler carrier rebuild the slides and rebuild the hyd track tensioners due to excessive stretch the tensioners exceded max travel the fr idler slopped back and forth breaking the main bolts that keep everything compressed ……. $$$$$$ ….. BUT the machine relatively a low hr (6500hrs) unit that had a lot of tracking time traveling …. just the cost of doing business !!!
Hope these pointer help a bit too !!! …. Cheers … GK