David De Gea went off injured with Manchester United 2-0 down to Southampton on Sunday afternoon after colliding with the post.
It took place 12 minutes before the break and Henderson came on at half-time to replace the Spaniard.
It didn't look good for De Gea who could face a long-time out if his knee injury is serious.
De Gea has lost his place for Spain and now Dean Henderson could step up and show his worth for Manchester United.
We will await word from the United doctors before jumping to any conclusions about the gravity of the situation.