Hint: where in the proof do we use that we apply path compression and how is that knowledge used?
Use for instance a potential function where the potential is the sum of potentials associated with each node, and each node x has a potential proportional to the logarithm of the size of the tree rooted by x. In the analysis of find, it is essential to distinguish between nodes fulfilling the usual union-by-size criterion and nodes which do not fulfill this criterion.