Lackey will be representing the Red Sox and Bay will be playing for the Mets, facing his former team for the first time in a game that starts at 1:10 p.m. ET and can be watched live on MLB.TV.
Keep in mind that things easily could have wound up the other way around.
If not for some medical differences of opinion between the Red Sox and Bay -- which started around the All-Star break and never really got resolved -- they likely would have come together on a new pact.
Once Red Sox general manager Theo Epstein found it blatantly clear in December that common ground could not be reached with Bay, he immediately swooped in and signed Lackey for five years at $82.5 million.
Before the Red Sox landed the big right-hander, the Mets were one of the teams most interested in plucking his services off the free-agent market.
While the Red Sox turned to an arm in Lackey after moving away from their big bat, the Mets did the opposite, re-allocating their resources to bring in Bay's All-Star slugging capabilities.
The Bay situation took a little longer to evolve, but he signed his new deal with the Mets in early January.
Now, Bay and Lackey will meet up in an exhibition environment.
In typical Bay fashion, he has taken the high road every time he's been asked about his parting of ways with the Red Sox, the team he thrived for in his brief (season-and-a-half) but memorable stay.
"It's the nature of the beast," Bay said recently. "No one owes you any favors. It's a business. You go out and you go one direction. I understand that. I liked [Boston] -- it was familiar to me, which was big. But I think being there prepared me to be [in New York]. I had a good time there, but I can also use it as good background for my time [with the Mets]."
Much like Bay has blended in seamlessly during his first Spring Training with the Mets, Lackey has done the same in Boston's camp.
"Honestly, it's been pretty easy," Lackey said. "It's been easier than I thought it was going to be. Guys have been great. It's a great group of guys -- easy to fit in with. I thought it was going to be more difficult than it actually has been."
This will be Lackey's second start of the Grapefruit League season. The first one was against the Twins, and Lackey retired all six batters he faced. Look for him to go three innings in this one.
Bay is 2-for-12 thus far, but has plenty of time to get in a groove before Opening Day.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.