We had a similar situation on our 268. After cruising for several miles we stopped, tried to shift to reverse, and the boat moved forward. The shift cable housing had come loose from the gear shift quadrant. In our case the nut came off the screw that holds the plastic bracket and cable housing to the quadrant. We were able to reconnect it with available hardware on the boat.

You don't say whether its Merc or Volvo. This may or may not be an issue depending on which end of the cable the plastic piece you mentioned is attached to. I am goingout on a limb and guessing it is at the throttle quadrant end. This would mean that the plastic piece is supplied by the throttle quadrant manufacturer. The plastic piece is most likely a Teleflex part that is part of the throttle quadrant assembly. On mine there were 3 pieces, the U shaped channel about 2 inches long that the cable housing fits in, a plastic plate about a half inch square that fit in a slot in the plastic channel and had a U shaped slot in it to fin in a groove in the cable housing, and a sheet metal U shaped cover that snapped in place on the 2 inch long plastic channel. Hope this helps.

Thank you! It is a Volvo. I wish I was more mechanically inclined. I will pass along this info, thanks again! Does anyone have any idea where I can purchase just throttle linkage?
The engine serial number for my Volvo Engine 401200B515 and 3869060 are both stamped on the engine