When Lord Visnu appears as a servitor, He is called an incarnation of a devotee of Lord Krsna

Expressions researched:
"When Lord Viṣṇu appears as a servitor, He is called an incarnation of a devotee of Lord Kṛṣṇa"

Sri Caitanya-caritamrta

CC Adi-lila

Although Śrī Advaita Prabhu belongs to the Viṣṇu category, He displays servitorship to Lord Caitanya Mahāprabhu as one of His associates. When Lord Viṣṇu appears as a servitor, He is called an incarnation of a devotee of Lord Kṛṣṇa.

CC Adi 6.112, Translation and Purport

The original bhakta-avatāra is Saṅkarṣaṇa. Śrī Advaita is counted among such incarnations.

Although Śrī Advaita Prabhu belongs to the Viṣṇu category, He displays servitorship to Lord Caitanya Mahāprabhu as one of His associates. When Lord Viṣṇu appears as a servitor, He is called an incarnation of a devotee of Lord Kṛṣṇa. Śrī Saṅkarṣaṇa, who is an incarnation of Viṣṇu in the spiritual sky known as the greater Vaikuṇṭha, is the chief of the quadruple incarnations and is the original incarnation of a devotee. Lord Mahā-Viṣṇu, who is lying on the Causal Ocean, is a manifestation of Saṅkarṣaṇa. He is the original Personality of Godhead who glances over the material and efficient causes of the cosmic manifestation. Advaita Prabhu is accepted as an incarnation of Mahā-Viṣṇu. All the plenary manifestations of Saṅkarṣaṇa are indirect expansions of Lord Kṛṣṇa. That consideration also makes Advaita Prabhu an eternal servitor of Gaura Kṛṣṇa. Therefore He is accepted as a devotee incarnation.