If you need a unit to stay on the board (e.g., a defending Ravenbloom that needs +1 might), can you play Retreat without targeting it or without recalling it?

Ruling: You must target a friendly unit if you have one, and if that unit is still on the board when the spell resolves, you must return it to hand. You cannot avoid targeting or recalling to keep a unit on the board.

Sequence:

• You must target the unit (if it's your only friendly unit)
• Upon resolution, if the unit is still on the board, you must return it to hand

Nuances:

• The silver rule (do as much as you can) applies, meaning you cannot choose to do less than what is possible