The phrase "being instrumental" implies an active role in achieving a certain goal, often through influence or contribution. Antonyms for this phrase could include "passive," "inactive," or "uninvolved." These words suggest a lack of agency or intentionality, and may indicate a role of support rather than leadership. Other antonyms might include "detrimental," "hindering," or "counterproductive," indicating a negative impact on a particular undertaking. Ultimately, the choice of antonym will depend on the context in which the phrase "being instrumental" is used, and the degree to which active involvement is valued or necessary.