You cannot pardon someone who hasn’t been tried and convicted.
If Manafort doesn’t spill the beans and is convicted, for Trump to then pardon him would be seen as two fingers to all internal and overseas security forces as well as the judicial system. Trump may want to remember this if he needs the Supreme Court to push through his legislation. But then questions will also be asked as to why he is pardoning someone convicted of such crimes?
So you then look at IF Manafort will try to cut a deal and why.