Re-attaching a radiator with thermostatic valve?

As part of some other work we had a leaky radiator valve replaced and the whole radiator was then left off for decorating etc. Now we want to put the radiator back on and the plumber said it would be a simple job. However I do not know how to replace the turn valve he left on there (I assume to avoid leaking) with the new thermostatic valve before I attach the radiator. Will the whole system have to be drained down? Or should I just call him back to do it?

Any help greatly appreciated and pics included:

https://imgur.com/a/ZgDoZvv