fangraphs says Senzel still has options and can be sent to minor league camp while remaining on the 40-man.
This is difficult to figure out, but IanRubbish made a point that the Reds released Senzel, thus making him a free agent. Because he was a free agent, his minor league options were voided and the Nats gave him a major league deal. He can't be optioned to the minors, and must pass through waivers and accept a DFA to stay in the organization.
He is only making 2Million, but I highly doubt ownership would cut ties this early. If he sucks in two months time, he gets cut. Gallo probably has a longer leash because he has a larger contract, but could also be trashed after the trade deadline if he still sucks.
Maybe we are actually going to see Senzel play the OF and Lipscomb at 3B? Then when Garcia Jr. Eventually fails, he gets cut, Lipscomb moves to 2B.
My guesses here are, in order of shortest leashes:
1. Garcia Jr.
2. Robles
3. Senzel
4. Gallo