* Apathetic: Lacking interest or enthusiasm.
* Indifferent: Not caring one way or the other.
* Uninterested: Not showing any interest.
* Disenchanted: Disillusioned or disappointed.
* Uninspired: Lacking in creativity or excitement.
* Passive: Not taking an active part or showing any interest.
* Unmotivated: Lacking motivation or enthusiasm.
* Lackadaisical: Showing a lack of enthusiasm or effort.
* Dull: Not interesting or exciting.
* Tepid: Showing little enthusiasm or interest.
* Reluctant: Unwilling or hesitant.
The best antonym to use will depend on the specific meaning and context of "enthusiastic" in your sentence.