The phrase "standing in lieu of" implies the concept of substitution or replacement. Antonyms for this expression could include words such as "original," "authentic," or "genuine," which all suggest a lack of replacement. Other antonyms could be "absent," "missing," or "removed," which indicate that something is not in its proper place, rather than filling in for an absent item or person. Additionally, words like "irreplaceable" or "indispensable" could be seen as antonyms, as they suggest that something cannot be substituted or replaced by anything else. Conversely, synonyms for "standing in lieu of" might include phrases like "taking the place of" or "substituting for.