The 28-year-old Miami native had his batting average dip to .197 after going hitless in three at-bats in a 2-0 loss to the Indians at Progressive Field.
A corresponding move will be announced on Sunday. The team has not officially said who is coming up, but it is expected to be outfielder Chris Coghlan, who was optioned to New Orleans on April 27.
"We just felt like we needed to send [Sanchez] down to get him going," Marlins general manager Michael Hill said. "He's not a .190 hitter. We do not think he's a .190 hitter. We think he's better than he's showing here. He's pressing.
"We want to take some of the pressure off of him, and get him down in Triple-A and try to get him right."
Manager Ozzie Guillen has three options to play first base in place of Sanchez.
Logan Morrison, the starting left fielder, has played first base most of his career. Greg Dobbs and Austin Kearns also can fill in at the position.
In 36 games, Sanchez has one home run and 11 RBIs.
A year ago, he was off to a terrific start. Sanchez batted .293 with 13 home runs and 50 RBIs in the first half. He was selected to represent the club in the All-Star Game.
But after the All-Star Break, he batted .225 with six homers and 28 RBIs.
"You're always evaluating your players," Hill said. "We know that he's struggled in the second half of last year, after an All-Star first half. So that's always in your evaluation. But we just saw a player who was struggling in the Major Leagues, and not the same player historically.
"We have first base options. We have Morrison, obviously, who can come from left field. We have Greg Dobbs, who obviously, can play over there, and we have Austin Kearns as well. We have guys who can play there."
The Marlins are weighing other roster decisions.
Center fielder Emilio Bonifacio had his string of 112 consecutive games snapped on Saturday due to a sprained left thumb.
Bonifacio had an X-ray and MRI taken on Saturday, and it revealed no fracture. The test results are being sent to Miami physician, Dr. Lee Kaplan, who will examine the center fielder on Monday.
There is a chance Bonifacio may go on the disabled list. Bryan Petersen has been playing center field, and Coghlan can play center.
"We're going to make a call to see how long the rehab time will be," Hill said. "If it's significant, then you make a move. If he can play through it, you try to avoid the DL, if you can.
"The results of the MRI are being sent to [Dr. Kaplan]. If he has conclusive information, then we can make a call."
The Marlins are in the thick of the division race, but they've struggled to score runs.
"We see a lot of guys struggling, pressing, trying to do more," Hill said. "Hitting is contagious. There is no lack of effort. No lack of working hard on these guys part. It's a combination of guys pressing, and we're facing some great starting pitching, there is no question about it."