It is no big deal to replace this seal by removing the upper cover if you know the key. Get the recommended sealer from MB. It will let you slide the cover rearward without disturbing the lower U Seal gasket. Put the seal in the cover on the bench, then oil the seal. While sliding the cover rearward use your fingernail to see that the seal is starting over the shaft smoothly. Push the cover rearward into place and bolt in place.
Gob the sealer in the lower rear corners, and of course, use it on the U Seal and rear sealing surface.
Nothing to it if you're using the right stuff.