When hull freezes over relegation beckons without star duo

A clever reference to the famous phrase "When the Thames freezes over, Hull will be relegated"!

For those who may not know, the phrase is a humorous way of saying that Hull City AFC (the football club) will be relegated to a lower division if certain conditions are met. The "Thames freezing over" is a metaphor for an extremely unlikely and improbable event, implying that Hull's relegation is just as unlikely.

In your version, "When Hull freezes over" is a playful way of saying that the team's relegation is imminent, but only if their star duo (presumably key players) are not performing well or are absent. It's a lighthearted way to express concern about the team's chances without being too dire or pessimistic.