Yes but it's tricky to pull off
This shield needs to do a couple things to be worthwhile.
1. Freedom of movement: It needs to be segmented to support movement while stowed on the back. Humans get away with strapping shields to our backs because the shield only attaches at two points. This shield will have continuous contact. If not segmented then the creature will need to be molded in such a way that this big plate doesn't impede movement.
2. Rapid Reattachment: The shield needs to support reattachment on the speed of seconds. Anything longer will require regrowing things and that takes too much energy (plus added risk of infection. Gross!)
3. Rapid Regeneration:Rapid hook-up to blood supply. This shield will need to regrow after being used. Blood supply means the shield can be a living thing. You'll probably want to design a crazy strong immune system to handle the inevitable infections. Also, this is totally unprecedented since there's no organism I know of that has what amounts to a external hydraulic valve for blood.
Segmented Shield, Detach!
A couple things need to happen to make the shield useful.
1. Detach blood supply of shield from creature. Probably want to flush the blood from the shield back to the creature. This prevents blood loss when the shield is damaged and gives the creature a kind of battle-boost of blood.
2. Detach shield holding mechanism. The mechanism could be a bunch of gecko foot pad like things that grip or intertwine with the shield to hold it in place.
3. Make the shield rigid. 548
There are a number of ways this can be done. It can be made rigid by attaching it to the forearm of the creature.
4. Rigidity would then come from the bones in the forearm though this seems dangerous to me unless the forearms are also strengthened for the added load.
5. Alternatively, have each plate interlock with it's neighbors. When there's blood flow to the shield, the space between the plates are pressurized, forcing them apart. This greater distance increases the range of motion. When there's no blood, the plates are elastically drawn back together, making the whole shield rigid. If the pressurized shield is hard to imagine, think of a spinal column. All the vertebrae interlock. Normally, they have a pretty small range of motion ~18 degrees. However, if you increase the space between each vertebrae, the range of motion is increased.
6. Clean up the cilia that hold the shield in place when not in use.
This is cool, why tricky?
It's awkward because usually you want armor all over. Also, there's no precedent for this anywhere in real life. You want armor to stay attached for the life of the creature. If additional armor is required, it just grows in that location. There are a lot of bioengineering challenges that will need to be overcome to make this work. The creature would need to be designed specifically to its shield. Also, all the tissue in the shield will need to be able to thrive in an an anaerobic environment for hours or days when the blood supply is cut-off. This likely means that regrowth will stop during that period. 671