Check engine light- exhaust manifolds

Hey guys. The check engine light came on in my 2008 cayman s. Mechanic says I’m definitely going to have to replace the exhaust manifolds. He says the problem I might run into buying used exhaust manifolds is that the check engine light could possibly come back on due to an insufficient amount of material inside the catalytic converter. The OEM new converters are full and will definitely fix the problem but used manifolds may not.

Can anyone confirm or deny this? There is a used set I’m looking into with about 75k on them.