This psychiatrist is only partially correct. The 'new' pathways are really existing pathways that are reassigned to new tasks. Since concussions have minimal focal injuries compared to a stroke or penetrating injury, a broader area of minor injury that does not necessarily lead to cell death is common. This often results in struggling cells due to out of control neurofibrillary tangles. Some people's brains correct this issue better than others. Treatments are being sought to correct this but they look to be expensive and years away. There are other broadly based non-cell death issues that can arise, too.

As you say, the challenge is to learn to make up for automatic processes with newer learned processes. We often call these work-arounds and accommodations. Accepting the new person is the start to learning these work-arounds. Trying to force or struggle through to use past processes can be highly stressful.