Josh Beckett had just pitched a no-hitter for the Dodgers and Ryu was scheduled to face the Cincinnati Reds on Monday night.
"You've got to throw a perfect game," Mattingly recalled telling Ryu, "to top Beckett."
Ryu almost achieved the feat.
The left-hander pitched seven perfect innings before surrendering a hit in the Dodgers' 4-3 victory over the Reds at Dodger Stadium.
A day after Beckett recorded the first no-hitter for the Dodgers since 1996, Ryu came within six outs of the first perfect game by a Dodger since Sandy Koufax against the Chicago Cubs in 1965.
But Ryu gave up a leadoff double to first baseman Todd Frazier on the second pitch of the eighth inning.
"Of course it was in the back of my mind," Ryu, through an interpreter, said when asked about the chance for a perfect game.
Ryu added that "it wasn't until the seventh inning that I thought it could actually happen."
The announced crowd of 45,505 gave Ryu a standing ovation after Frazier's hit. He received another when Mattingly replaced him with Brian Wilson after the Reds sandwiched a run-scoring sacrifice fly between two singles.
Catcher Drew Butera caught Beckett on Sunday and Ryu, for the first time in a game, on Monday.
"He made my life easy," Butera said.
Before Monday's game, Beckett sat in the dugout and said pitching a no-hitter still felt "surreal."
That was an apt description of the vibe in Dodger Stadium as Ryu dominated the Reds.
He struck out at least one hitter in every inning except the fourth, when third baseman Justin Turner, who made an outstanding play in the first inning, made two more.
Ryu came out to pitch the eighth against the middle of the Reds' lineup. Frazier had hit grounders to Ryu in his first two at-bats. This time, he took a pitch and then hit a line-drive double to left field.
Ryu's performance overshadowed pregame drama.
As the Dodgers opened their homestand, it remained clear that the ebb in the team's lineup flow, and perhaps the drama surrounding Kemp, would continue indefinitely.
Of course, if the Dodgers continue to get the kind of pitching they had the last two days, it might not matter what happens with the lineup.
Ellis said he jumped to join the dog pile in Philadelphia and his right foot landed on Butera's mask.
"Beyond frustrated," Ellis said in the clubhouse before Monday's game. "Still kind of shocked."
Ellis was put on the 15-day disabled list and Tim Federowicz was recalled from triple A.
The Kemp situation is potentially more delicate.
Mattingly said last week that Kemp, a two-time Gold Glove winner, would begin working in left field. Kemp, who is being paid about $21 million this season, underwent off-season ankle surgery and has not shown the burst that helped make him one of the most dynamic players in baseball.
"It's frustrating any day you're not playing," Kemp said. "I'm here to play baseball. I'm here to try my team win anyway possible."
Kemp has not played left field since his rookie season in 2006. He said his ankle was sound and he still considers himself a center fielder.
Asked whether he thought he was being treated unfairly, he said, "It doesn't matter what I think. I just want to go out there and play baseball, man, that's all that matters."
Mattingly said Ethier was the center fielder for now.
"We feel like he's the best option right now," he said.