If Fukushima were Chernobyl, it would have needed a sarcophagus. However, it was already designed with containment that worked, and as a result, a sarcophagus wouldn't have changed anything. In fact, building a large concrete structure over the reactor core would probably have complicated cleanup efforts by limiting access.
In effect, Fukushima already had a sarcophagus integrated into the structure of the building in 1971, 6 years before Chernobyl even started running.
I don't mean to suggest that Japan should build a sarcophagus; I only mean to give an idea of the sort of timescale for significant engineering tasks that the Soviets were working with. They slammed it into high gear pretty rapidly.