Reported speech- could
How to rewrite this sentence to reported speech?

The manager will be here tomorrow if you could come back then.
The assistant said the manager............................

How should I rewrite could? "could have +past participle"?

Question

Reported speech- could
How to rewrite this sentence to reported speech?

The manager will be here tomorrow if you could come back then. 
The assistant said the manager............................

How should I rewrite could? "could have +past participle"?

Answer

No - it stays the same (just "could")

Answer

The assistant said the manager would be there tomorrow and asked if I could come back then

Answer

It all stays the same if you're reporting it before you go.  The situation hasn't changed.
Also, the idea that you have to change tenders when you report speech is wrong.  It depends on the situation, add you can see from your example. 
:)

Answer

The assistant said the manager could come back then.
This means the assistant said it is possible that the manager will come back.
If the assistant was sure that this will happen then use 'would' instead.