Peter King puzzler

The Pirates are starting to get some respect from some national media guys.

Exhibit A: SI's Peter King writes:

b. Two quick non-Sox observations: PNC Park is such a gem. And Rob Mackowiak is the next Travis Hafner.

In one corner, we have Rob Mackowiak. In the other, Travis Hafner.

I like the comparision. But what does it mean?

(Thanks to reader Dan.)