The antonyms for the word "long-serving" are varied and can include terms such as short-term, brief, fleeting, momentary, temporary, and transitory. These words suggest a lack of longevity, permanence, or continuity. They denote a shorter period of service, indicating a limited duration of time spent in a particular role. Other possible antonyms could be sporadic, intermittent, occasional, or isolated. These terms indicate little or no consistency, suggesting that the service or tenure is not steady or continuous. Ultimately, these antonyms serve to highlight the temporary or limited nature of the service or position held.