If Indianapolis were lost before Tinian

What if USS Indianapolis had been sunk on the way to Tinian?

Another heavy cruiser would likely repeat the delivery with an escort, delaying the end of the war by about a month.

The key effect would be delay rather than permanent failure of the mission. A replacement heavy cruiser would have to run the route again. The second attempt would likely receive destroyer protection against submarines. The atomic-bomb delivery and therefore the war's closing timetable might slip by roughly a month rather than collapse entirely.