I think the real question you're trying to ask here is: **How can I avoid letting my players take free long rests whenever they want?**

In the situation you described, your group felt free to use lots of resources on a minor battle, because they knew they could just rest right afterward.

There are two approaches I use to avoid this.  

 - One approach is *time pressure*: if the group is on a time limit,
   that gives them a finite number of rests they can take before.

  

 - The other approach is *wandering monsters*: if the group tries to
   rest in a dangerous area, I make up an encounter (or I have an
   encounter prepared) and I use that.

In either case, I try to communicate the time limit or the environment danger clearly, so that characters don't overuse resources on early encounters.

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To answer your question literally: you need to have a clear reason why the characters can't just long-rest whenever they want, and you need to communicate that in advance so that they know to conserve their daily powers.  If they overuse their daily powers and *then* you decide their actions should have consequences because that wasn't what you wanted, they'll feel that you're being unfair.