When describing two distinct events that happened in the past, standard English grammar uses the past perfect tense (had + past participle) for the event that happened first, and the simple past tense for the event that happened more recently. In this context, the exhibit deteriorating happened first, and the curator replacing it happened afterward. Therefore, 'had deteriorated' (past perfect) is correctly paired with 'replaced' (simple past) in the sentence: 'The curator replaced the exhibit after it had deteriorated.'