CHICAGO -- After a sequence of unpredictable events, the Marlins and Padres on Monday agreed it was in the best interest of both squads undo a two parts of last Friday's seven-player trade.
Before the 4 p.m. ET non-waiver Trade Deadline on Monday, Miami returned right-hander Colin Rea to San Diego for pitching prospect Luis Castillo. The improbable deal came after Rea injured his elbow on Saturday in Miami's 11-0 win over the Cardinals at Marlins Park.
Rea, making his Marlins debut, threw 3 1/3 innings before exiting with a sore right elbow. On Sunday, the 26-year-old was placed on the disabled list with a right elbow sprain. A day later, he was heading back to San Diego, and he was scheduled to have an MRI.
"In the end, we just felt like it was in the best interest of both clubs to make the trade, Colin Rea for Luis Castillo, and move forward," Marlins president of baseball operations Michael Hill said. "I really don't want to get into his situation."
Castillo, a hard-throwing 23-year-old, will return to Class A Advanced Jupiter.
In Rea, the Marlins had initially hoped they had addressed their fifth starter.
In Friday's trade, Rea, Andrew Cashner and pitching prospect Tayron Guerrero were dealt from San Diego to Miami for right-handers Carter Capps and Jarred Cosart along with prospects Castillo and first baseman Josh Naylor.
"When we made the trade, there were no concerns there," Hill said. "Today, we just felt like it made the most sense to make the deal to return him back to San Diego and get Luis Castillo back to the Marlins."
Before completing Friday's deal, the clubs reviewed medical records.
Rea had previous elbow soreness, stemming back to late last year, when he was shut down on Sept. 8.

Hill acknowledged the Marlins were aware of his past elbow issue.
After dealing Rea back to San Diego, the Marlins continued to explore a deal for a starting pitcher, and they also inquired about some left-handed relievers, including Hunter Cervenka from the Braves.
But the Marlins were unable to find a match, and the Deadline passed without any further acquisitions.
"I don't think anything changed," Hill said. "I think, all-in-all, we were very happy with what we were able to accomplish, leading up to 4 o'clock today."
The Marlins on June 30 acquired setup reliever Fernando Rodney from San Diego prior to getting Cashner.
"We added a quality back-end reliever in Fernando Rodney, and we were able to add, in our opinion, a tremendous starter in Andrew Cashner," Hill said.
The Marlins still are weighing their fifth starter options, and are exploring the waiver wire. But, for now, they are prepared to address their rotation internally.
David Phelps, who has been used primarily in a setup role, is the frontrunner to join the rotation.
Phelps has already thrown 54 1/3 innings in 50 appearances, but he's made 59 starts in his career, including 19 last year with the Marlins, and he is 16-19 with a 4.36 ERA in that role.
On Saturday, Phelps worked 2 2/3 innings of relief for Rea, throwing 55 pitches.
"We're definitely thinking about Phelpsy," manager Don Mattingly said. "We haven't made a decision. Obviously he's down for a few days from that outing the other day."
Phelps would have to be built up, pitch-count wise. So his next time out, he could go about 70 pitches.
The Marlins opened a three-game series at the Cubs on Monday, and they are off on Thursday before facing the Rockies at Coors Field. The club has yet to determine its starter for that night, while Cashner is lined up for Saturday.