@Zombie Well, the 'e' irritates my inner red pen. Nouns can be adjectivised by adding an 'ig', but that almost always calls for the red pen again 'tischig', 'ersatzig' *shudder*. The chair looks 'tischig' (like a table), tablish? Everyone would know what I mean, but grammatically, it would be horribly wrong. If we change the word form it becomes a different word, too, in this case ersatzslos, (without replacement) or ersetzbar (replaceable). And then we have our famous compound words, Ersatzmann, or in @Mrsgnomie's case, Ersatzmilch.
Thank you for the explanation, Zombie ,but in this case, I think I will avoid ersatz.