"Scunthorpe are a really hard working team. They fight for each other and won't make it easy," said King, who has recovered from flu to take his place in the matchday squad.
"They will see no reason why they should not beat us. Hopefully we can do our jobs and win."
Watford are among the favourites to win promotion this season, and couldn't have asked for a better start.
But despite sitting pretty in the table, King sees room for improvement in the side.
"It's strange. We are top of the league but we have still not hit full potential," he insisted.
"We are winning ugly. It's exciting to think how good we might be when we start playing well. It's a tough league though.
"There are no bad teams and there isn't any ground you can go where you can expect to win easily."
Centre-back Andy Butler is a doubt for the Iron with a dead leg, with winger Kevan Hurst also looking to shake off a knock.