Young returned from the same anterior capsule surgery as Santana, only two months later, giving him eight less starts and New York no clear picture of consistency.
Chacin returns from more than two months on the disabled list with right shoulder inflammation, with Colorado eager to see whether the 24-year-old shows the promise he did the past two seasons, or the struggles he did in five starts this year.
"It's placing a guy that's been on the disabled list for a long period of time and putting him back in the rotation to see how he goes more than anything else," Rockies manager Jim Tracy said of the roster move his club will make prior to the second game of a four-game series at Citi Field.
The past year gave the Rockies little idea of what type of pitcher Chacin is.
The right-hander went 8-7 with a 3.16 ERA in 18 starts before the 2011 All-Star Game, but he is 3-10 with a 5.03 ERA in 18 starts since. He posted a 2.63 ERA in two rehab starts with Triple-A Colorado Springs, most recently throwing seven innings of one-run ball, but he walked three batters and struck out none in that start.
Now he returns to the rotation attempting to prove he is still the pitcher who posted sub-4.00 ERAs in his first two seasons as a starter.
Young pitches with a similar burden. At times since his return he fooled opponents with his 6-foot-10 frame and deceptive delivery. He held the Giants to one run in seven innings on Aug. 2, and held the Reds scoreless, despite 12 batters reaching base, in his last start. In three other starts, he didn't complete five innings.
While the Mets might shut Santana down with the first half of his season as proof he can return to some semblance of his All-Star form, Young still lacks the consistency the Mets saw for all of four starts before his 2011 season ended with injury.
Now with their teams out of contention, Chacin and Young are pitching for next season as much as they are for an August matchup in New York.
Colorado: Decision on Young due Tuesday
Eric Young arrived at Citi Field on Monday feeling much better than he did a day earlier, when he left the Rockies' game against the Marlins in the first inning with a left intercostal muscle strain.
Young said Monday a trip to the disabled list was unlikely, but Tracy was less committal, saying a decision would be made for sure before Tuesday's game. The Rockies will have to make a roster move to activate Chacin from the disabled list.
"He has a tendency to respond very quickly, and yet this situation with regard to this type of injury is one that doesn't cooperate for a player too well," Tracy said. "If he's still sore [Tuesday], then we've got to figure something out."
Charlie Blackmon led off in place of Young on Tuesday, going 0-for-4, and Dexter Fowler played center field.
New York: Santana slated to start finale against Rockies
While the rest of Santana's season is shrouded in uncertainty, the left-hander will make his start Thursday against Colorado at Citi Field as long as his back allows him to do so. Some recent back discomfort will force Santana to undergo an MRI on Tuesday.
He could make another start next week in Philadelphia, but after that, the Mets are considering shutting him down in his first season back from a year on the DL.
"At some point, there's a diminishing return," said general manager Sandy Alderson. "It's been a long year, a long offseason of rehab. Johan actually has accomplished quite a bit over the course of this season, and at the appropriate time, we really have to think as much about next year as we do about this year."
The 33-year-old threw a shutout on May 26 against the Padres and a no-hitter in his next start on June 1 against the Cardinals, but he is 0-5 with a 15.63 ERA in his past five starts.
Carlos Gonzalez will be unavailable again Tuesday, as he will arrive in New York late at night from his native Venezuela, where he is mourning the death of his grandfather. The Rockies expect to activate him from the bereavement list Wednesday.
German Rosario, a 16-year-old Dominican shortstop the Mets signed to a $1.75 million contract last month, took batting practice at Citi Field before Monday's 3-1 loss to the Rockies.
Mets outfielder Mike Baxter extended his hitting streak to eight games with a third-inning single Monday. He went 2-for-5 to improve to 11-for-30 since Aug. 9.