How do you deal with engineers with performance issues?
Here the manager should work on creating the performance improvement plans with the agenda of getting or bringing that engineer back to the speed needed for this project. A manager needs to work closely with that engineer and track consistently how things are going as per the plan. The manager needs to provide all kinds of support that engineers needed at that time.
If the manager thinks or found no improvement in that engineer, he should look out for other positions in the company. If nothing is available in the company according to that engineer’s skills, he should be counseled out from the company.