Promising the team a win is easy. Promising them your effort is the one you can actually keep.

Outcomes depend on other people, luck, and a hundred things nobody controls, so promising one is borrowing against a future you cannot see. Effort is entirely yours, which makes it the only promise you can actually honour. It also happens to be the one that holds a group together when results go badly, because it is still true after a loss.

Say what you will do rather than what will happen. Before the next big thing, tell people the specific commitment you are making and nothing about the result. Then keep it in the week where it is inconvenient.