That football team was once reputed to be invincible.......

once雖然就咁解係「一次」
但用o係句子中作助語就解作「曾經」
例子：
The retired actress was once famous for her beauty.
He was once the top of the class. After getting addicted to computer games, this was no more.
通常once都放o係verb to be後面

once 在這句中應譯作「曾經一度」
這是強調表明那事件只維持了一段很短的時間，就完了！
若然不使用 once 這個字，這句就會變成：「那隊欖球隊從前被視為長勝不敗之兵。」含意因此少了一截！