Major League Baseball instituted the new paternity leave list this season -- players are allowed to go on leave for 24 to 72 hours. It remains to be seen whether Bay will play in Wednesday night's and Thursday afternoon's games, also against the Giants.
"I don't know right now," said Mets manager Terry Collins on Tuesday afternoon. "I talked to Jason late last night, and it sounds like it's gonna be two [days]. But I'm not sure."
To take Bay's place on the active roster, the Mets promoted outfielder Lucas Duda from Triple-A Buffalo. He will be in the lineup on Tuesday night against righty Ryan Vogelsong, batting sixth and playing left field.
Duda, 25, began the season with the Mets, but was sent down to Buffalo on April 10 after getting just two hits in his first 17 at-bats (.118), with no homers and one RBI. In 21 games at Buffalo, Duda is hitting .257 with three homers and nine RBIs, including five multihit games in the team's last eight games.
Bay, 32, began the season on the disabled list with a strained muscle in his rib cage. He was activated on April 21, and is hitting .250 with one homer and three RBIs in 40 at-bats.
This will be the third child for Bay and his wife Kristen.
Kieran Darcy is a columnist and reporter for ESPNNewYork.com.