Hornets boss Ray Lewington said he thought Southampton would probably be delighted to draw his First Division side.
He said: "I should imagine the Southampton lads will be jumping up and down.
"We were the side everyone wanted, but we have sprung surprises before and there may be another one around the corner."
In the other semi-final First Division rivals Sheffield United will play either Chelsea or Arsenal.
Both semi-finals take place on April 13th.