Alvarez, who came to the Marlins in last November's big trade with Toronto, had been expected all week to make his season debut Thursday night at Turner Field. With the day's transactions, he officially came off the 60-day disabled list. He had been placed there after suffering inflammation in his throwing shoulder during spring training.
"There's a little bit of the unknown with him because we didn't get to see him much before he got hurt," Marlins manager Mike Redmond said. "I know in seeing him the few times that I have when he was in Toronto, this is a young kid that can throw strikes and eat up a lot of innings.
"He can win games and will be a big help as far as another young guy, another young arm that knows how to pitch."
Koehler, who got the no-decision in Tuesday's 11-3 loss to the Braves, goes down with a 1-5 record a 4.70 ERA in 10 starts with the big-league club. He spent some time pitching out of the bullpen while up with the Marlins, but Redmond said he would remain a starter in Triple-A.
"He did a good job while he was here," Redmond said. "He gives us some flexibility where he can start or pitch out of the bullpen, but he did a nice job starting and got some valuable experience."
Before Thursday's start, Alvarez had four rehab outings in Single-A Jupiter and Double-A Jacksonville. He went 2-0 with a 1.12 ERA and 15 strikeouts in those games.
Alvarez's injury came at the end of spring training. He had four starts for the Marlins in March, going 1-0 with a 3.55 ERA in them. He also pitched three innings of relief in the World Baseball Classic for Team Venezuela, giving up three runs, five hits and a walk and a hit batter in his lone appearance against Puerto Rico in San Juan, Puerto Rico.
The right-hander came into Thursday's series finale with a 10-17 career record. Alvarez spent the last two seasons pitching for the Blue Jays. When he earned his first career win at Baltimore on Aug. 31, 2011, he became the youngest Blue Jay to win a game since Kelvim Escobar in 1997. Alvarez was 21 years and 135 days old when he notched that win.
In addition to the news about Koehler and Alvarez, the Marlins announced that outfielder Matt Diaz was transferred to the 60-day disabled list. Diaz has been out since May 20 with a left knee bone bruise. He had previously been placed on the 15-day DL.