Will db2 be able to use the index if I don't reorg it following an inplace reorg?

If you couldn't use the index, then an inplace reorg would be useless.

DB2 documentation about inplace reorg:
Indexes are maintained, not rebuilt, so subsequent index reorganization might be required


might be required -> how do I tell if index reorg is required or not?

What they mean by "might be required" is that the indexes may be disorganized enough that they might perform better if reorg'ed. They are not suggesting that the indexes cannot be used by a query just because you did not reorg them.

This is in contrast to a offline reorg on a table, where the indexes are automatically reorg'ed (actually they are just rebuilt). So they are just trying to warn you that an inplace table reorg does not reorg the indexes.