I'm not sure what held the team back from selling any time in 2019 or before the 2020 season. Maybe elf is right that the team had no confidence in their ability to replace him like they have been with other high profile departures over the years.
I tend to think Thelwell doesn't have those sorts of nerves. If a player meets the criteria for sale (last year of contract and near to their highest projected market value), then I think Thelwell will sell and figure out how to repair the roster after. Right now, I'm surprised, but its looking like there won't be an insurmountable hole if Long is sold and he could just backfill with youth. Long injury aside, it seems like Thelwell's dream scenario - sell a player that you got for free and buy a couple kids to replace.
Loan with option to purchase makes sense in light of the injury. While I don't suspect we'll be able to cheer Long on in an RB jersey again, hopefully we get to in a Nats jersey.