Question:
Answer:
Not to worry. As the Mother of God—and our spiritual mother as well (Rev. 12:17)—Mary will always lead us to her divine Son. Recall her words at the Annunciation, “Let it be to me according to your word” (Luke 1:38), and also at the wedding at Cana, “Do whatever he tells you” (John 2:5). Mary is unfailingly faithful to Jesus, so to serve her is serve him and thus harmonious with the Gospel, including the passage you cited:
And Jesus answered him, “It is written, ‘You shall worship the Lord your God, and him only shall you serve.’” (Luke 4:8).
To be clear, we only worship God, but serving Mary in fidelity to Jesus is, again, not contrary to the Gospel.
In addition, there are more than one renderings of this promise of the 15 Promises of the Rosary, which Our Lady revealed to St. Dominic, to whom the Rosary was first given in the 12th century, and later to Blessed Alan de la Roche, who renewed devotion to the Rosary in the 15th century. One version places it as the first promise and renders it the way you have presented it: “Whosoever shall faithfully serve me by the recitation of the Rosary shall receive signal graces.”
Another version renders it without the words of service, yet implicitly makes the same point: “Those who shall persevere in the recitation of my Rosary shall receive some signal grace.” It also lists it as the second promise.