The 20-year-old right-hander made the Mets with a terrific spring and was used in relief before being sent to Double-A in late June to stretch him out to become a starter. He was 0-2 with a 3.25 earned run average in 30 appearances with the big-league team.
"It helped me," Mejia said of his time in the bullpen with the Mets. "Now I know I can pitch here."
Mejia excelled at Double-A Binghamton after being sent down, going 2-0 with a 1.32 ERA in six starts. In his first Triple-A start for Buffalo, Mejia gave up a run in eight innings on Monday.
He faced the Cubs twice earlier in the season, tossing two scoreless innings.
The Cubs drew first blood in this series on Friday, as Blake DeWitt's three- run homer in the sixth provided the difference in Chicago's 7-6 victory.
DeWitt hit his fourth home run in just 30 games with Chicago since a midseason trade with the Dodgers, and the Cubs improved to 7-3 under interim manager Mike Quade. Alfonso Soriano also hit a three-run shot in the victory, while Xavier Nady went 3-for-4 with an RBI and two runs scored.
Chicago's Randy Wells allowed four runs, eight hits and four walks in the start, and James Russell (1-1) earned the win for retiring the only batter he faced. Carlos Marmol ended up with his 26th save of the season.
David Wright hit a solo homer among three hits and drove in three runs for the Mets, while Luis Hernandez hit his first home run with the team. That still wasn't enough to prevent New York from falling for the fourth time in five games.
Mets starter R.A. Dickey (9-6) squandered an opportunity to set a new career- high in wins after giving up a season-worst seven runs on eight hits and a walk in six frames.
Today Chicago will hand the ball to volatile righty Carlos Zambrano, who has won his last three decisions. Zambrano won his second straight start Monday against Pittsburgh, which managed just an unearned run and four hits in 5 1/3 frames off him.
The three-time All-Star is 3-0 with a 1.84 ERA in five starts since returning to the rotation following anger management classes.
Zambrano lost to the Mets earlier in the year and is 4-3 lifetime against them with a 3.63 ERA in 11 games, 10 of which have been starts
The Mets won three of four meetings with the Cubs in a series held in New York back in April.