Preheat oven to 400 degrees F (200 degrees C). Place a pie dish onto a baking sheet.
Mix the strawberries, rhubarb, sugar, tapioca, and lemon juice in a bowl, and let stand for 15 minutes, stirring occasionally. Fit a pie crust into a 9-inch pie dish; pour the filling into the bottom crust. Scatter pieces of butter over the filling, and place the top crust onto the pie. Press the edges together and crimp with a fork to seal. Cut several slits into the top crust with a sharp knife.
Bake in the preheated oven until the crust is golden brown and the filling is bubbling, 40 to 55 minutes.