Hybrid solution: build a rotational secondary including safeties and corners changing roles
Another wilder solution would be using sophomore safety Kyle Hamilton in the slot. Baltimore did it last year, but that would open a hole in the deepest level of the secondary that another reserve would need to take, most probably Brandon Stephens.
Also, Hamilton was always expected to transition to full-time safety this year given his skill set and production at the position, something he's done in the first two games of the season to great results. The change might not be worth it.
Backup safety Geno Stone was sublime against the Bengals in Week 2 taking over Marcus Williams' vacated starting position, but putting Hamilton in the slot would mean using two backups (Stone and Stephens) as starters going forward. No bueno.
The cornerback rotation is missing Humphrey, which means that Rock Ya-Sin and Ronald Darby will keep getting most of the reps, with a dash of Stephens in a super-rotational role, and perhaps/maybe Kevon Seymour and/or Daryl Worley and Jalyn Armour-Davis.
At the end of the day, there is no optimal solution for Washington's injury because it has not come as an isolated problem. Washington's absence comes along with those of another starting cornerback and another starter in safety Williams.
That's three holes to fill in one fell swoop, which is unreasonable to think about as something easy to accomplish.
There are many solutions Harbaugh and defensive coordinator Mike Macdonald can go with in Week 3 and later, but none of them sounds like music to our football ears.
Macdonald offered the world a defensive masterclass on how to manage a banged-up unit last weekend, and he will have to keep on doing it for the time being.