The 19-year-old will remain at the Crown Ground until January.
Watson told Accrington's website: "I really wanted to get some league games under my belt.
"This is going to be very different to the reserves where I've been used to playing and hopefully it'll be a better standard for me to get some games."
Manager Paul Cook added: "People might look at our squad and think we've got a lot of numbers but there are certain areas where we don't have a lot of cover.
"Ryan gives us a little more cover and depth to the squad. With seven subs this year and inevitable injuries and suspensions we'll need strength and he's a great addition."