Sprinkler System Wire cut accidentally
Use a junction box anyway

The goal is to make the splices in dry space so they don't fail later.  The primary water-keeping-out strategy is to have the wires enter the bottom. Trying to watertight electrical boxes "like a boat" is generally a lost cause.  

Go to a real electrical supply house.  Get


an outdoor junction box with 2 entries on one side, ideally 4" x4" so you have extra room to work. 
A blank cover for the box 
Two strain reliefs of the correct size  for the cables. They should not be watertight strain reliefs.  And 
A few small (blue) wire nuts, ideally Ideal.  Don't over-buy the nuts, they have little use outside thermostat, doorbell and rewiring fluorescent lights. 


The reason to use a proper electrical supply house is the lid will fit the box, and the strain reliefs will be the correct size and type.  Also it will take 3 minutes instead of an hour. And will be cheaper. And won't involve bad advice! 

On the bench, plug and seal any holes not on the bottom with the supplied plugs and silicone. Strip wire ends.  Mount the box, fish the wires in with the strain reliefs, and make your splices.  Cover on, done!