Competency is not the same as experience but experience tends to improve competence. If Mamdani is to be successful he'll seek out experienced staffers to help him. That's not a knock on him, just reality -- it helps to have people who aren't in their first rodeo.
Dunno if Brad Lander would be willing to be Mamdani's Deputy Mayor for Administration and Chief of Staff, but that to me would be a great start.
That would be a great slot for him, and honestly, given how Lander campaigned, I'm guessing there's some level of quid pro quo at work here -- which would be smart for both men.