Maddux has swapped bodies with Mitre. It probably happened something like this.

Greg invited Sergio over to talk golf, or perhaps pitching. Over mimosas, "The Professor" drugged the young pitcher, and dragged him down to his hidden lab. There, Mitre was strapped to a table, and Greg's assistant, Henry "Igor" Blanco, then strapped the Professor down. Blanco limped over to the power lever and threw the switch.

Sparks flew, there was a low rumbling sound, followed by a high wailing sound that was similar to Ron Santo whining about the Cubs poor play.

Moments later, their minds were swapped. The veteran mind of the professor in the young, skilled body of Mitre, and vice versa.

I think the best evidence of this is the facial hair. As those long time Cubs fans might remember, Maddux was somewhat facially hair challenged, particularly early on. Now blessed with the ability to grow a groucho-esque bit of facial hair in "just a couple of days," he has jumped at the opportunity.

 It is the only solution.