This will probably get some clarification from Sage Advice before long.
Imagine the following scenario:
Round 1: Ranger commands pet to attack orc 20' away. Pet attacks. Orc counterattacks.
Round 2: Ranger fights a second orc that comes after him in person. Meanwhile, on the other side of the room, the first orc attacks the pet. Does the pet just sit there and take the hit because it hasn't been explicitly ordered to attack?
I don't think anyone can argue that the pet will just sit in place for several rounds while the first orc attacks it, never counterattacking on its own. In round 1 the pet got involved in a life-or-death struggle and its instincts will tell it to run away if outmatched or to finish the fight. The pet surely has enough intelligence to act on its own in this situation without direction. But I'd say it's under the DM's control at that point, not the player's control.
So if the ranger is distracted, a reasonable situation would be for the pet and orc to finish their fight, and then for the pet to return to the ranger's side or prowl the boundaries of the combat waiting for another command. Or if the pet gets heavily wounded, it might run away and return after the battle, or return to the ranger's side hoping for help.
On the other hand, the ranger could command the pet to do something else even while it's engaged in a fight. "Return to me", "Fight the ogre, not the orc", etc. It costs an action, but that's reasonable if you're forcing the pet to an action that overrides its instincts and habits.
There is obviously a bit of negotiation to be done between the player and the DM about this, but as I DM that's how I would run it. Spend your action to declare the pet's action, but if you don't spend your action it will behave like a normal animal.
The fuzzy region then becomes: how loyal is the animal to the ranger, really? Would it automatically defend the ranger, or try to save the ranger from death? That sounds like the kind of bond that might be worth expressing as an alternative class feature or a feat, but not necessarily the default state of a beastmaster. Training an animal doesn't mean the animal loves you and behaves altruistically towards you--and many species don't have the pack instincts that would make this kind of bond easy to create.