The adverb of 'unavoidable': in a way that could not be prevented. It tells you why something turned out as it did, and it is nearly always an explanation offered in advance of blame. Most of its real life is in a handful of set phrases, and those are what to learn: 'unavoidably delayed', 'unavoidably detained' (a formal way of saying someone is late and it was not their fault), and 'unavoidably absent'. All three belong to offices, letters, notices, and announcements — 'The chairman is unavoidably detained' is the classic. A second, quieter use puts it before an adjective or a whole clause to admit a weakness up front: 'the report is unavoidably long', 'unavoidably, some of the data is out of date'. Here the writer is saying, I know this is not ideal, but the alternative did not exist. In casual speech people simply say 'I couldn't help it' or 'there was no way around it'.