Rosario, 27, joins a Dodgers team whose .629 OPS at shortstop is the fifth worst in baseball and that just a day ago traded for super-utilityman Enrique Hernández, who had been the Boston Red Sox's shortstop.
Syndergaard, 30, will join the Guardians in Chicago on Thursday for the opener of a four-game series, but it's not clear when he'll make his debut, according to Chris Antonetti, Cleveland's president of baseball operations. He is rehabbing at Triple-A after hitting the injured list in early June because of a blister on his right index finger.
After signing a one-year, $13 million contract with the Dodgers in the offseason, Syndergaard posted a 7.16 ERA over a dozen starts, and the Dodgers no longer envisioned a long-term place for him within what remains a needy rotation.
The Dodgers' hope is for Rosario, who is hitting .265/.306/.369 with three home runs this season, to capture some of his previous offensive and defensive magic. Last year, he registered more than four wins above replacement, and he discussed a long-term contract extension with the Guardians before agreeing to a $7.8 million salary in arbitration.
Rosario is due to hit free agency this winter and will be among the youngest players in the class. He made his debut with the New York Mets at age 21 and was traded to Cleveland in 2021 as part of the four-player package for shortstop Francisco Lindor and starter Carlos Carrasco.
Cleveland could replace Rosario with young infielders Tyler Freeman and Gabriel Arias, and also could turn to 22-year-old Brayan Rocchio, who made his major league debut earlier this season and is currently hitting .295/.385/.419 at Triple-A.
"We felt that this made sense for us, and now is the right time to give Gabby and Tyler some more opportunities," Antonetti said. "We also wanted to be respectful to Amed. We recognize his place on our team and as a veteran leader and his expectations of himself and transitioning him to a lesser role would had an impact on him and the team and we were very mindful about doing that."
Cleveland's beleaguered rotation could get a boost from Syndergaard, the one-time All-Star who will join his fourth team in two seasons after getting dealt from the Los Angeles Angels to the Philadelphia Phillies at the trade deadline last year. Three of the Guardians' top starters -- ace Shane Bieber and right-handers Triston McKenzie and Cal Quantrill -- are on the injured list, and the team currently has three rookies (Gavin Williams, Tanner Bibee and Logan Allen) starting alongside veteran Aaron Civale.
The Guardians' pitching nonetheless has been among the team's strengths, with a 4.07 starters ERA that ranks 10th in the major leagues. Cleveland could continue to pursue a bat to enhance an offense whose 420 runs are 25th in the major leagues.
ESPN's Alden Gonzalez and The Associated Press contributed to this report.