If Sanders doesn't run himself, then what is the probability that endorses anyone during the primary campaign? There are certainly costs to doing an endorsement, as all of the candidates will want to suck up to him and pay lip service to his issues as long as he's officially "neutral".
If he endorses someone it will be Warren. He has said that he wouldn't have run in 2016 if Warren was running, and recently at a West Virginia rally he called her one of the greatest senators this country has.
But I don't think Bernie has given up on becoming president.