Mark Wright equalised for the Shrews after Asa Hall's opener for the home side but James Constable's second-half double against his former club won it for the U`s.
Turner said: "Goals change games.
"We gave them a goal start, got back into the game but the second and third goals were abysmal from our point of view.
"There were balls bouncing around the edge of the box, we've hesitated and paid a heavy price for it.
"We let everybody down, we've not performed. We were awful."