In the two-plus years since that Series, the right-hander had thought it was the former. He was rocked by the Astros that postseason while a member of the Dodgers, and Houston players told him afterward that he was tipping pitches.
But now, after a Major League Baseball investigation found that the Astros illegally stole signs that year, Darvish isn't so sure.
"That's what I want to know," he said Friday at the Chicago Cubs' winter fan convention.
"A couple of Astros players told me I was tipping pitches, but now it comes out they were stealing signs. Was I tipping, or were they stealing?"
Such questions have mounted around baseball since MLB commissioner Rob Manfred unveiled the league's report on the Astros on Monday.
For Darvish, it hit especially close to home. While with the Dodgers, he had a combined 1.59 ERA in the 2017 NL division and championship series. But against Houston in the World Series, his ERA ballooned to 21.60. He gave up four runs in 1.2 innings in Game 3 in Houston and got hit hard again in Game 7 in Los Angeles, taking the loss as the Astros captured their first World Series title.
That sequence of events left Darvish with more questions than answers.
"I know they were stealing signs, but at the same time, I was not good during the World Series," he said.
His World Series struggles likely had long-term ramifications as well. He was a free agent after that season, and the Cubs later admitted that teams might been scared off by Darvish's performance against the Astros, giving Chicago an easier path to signing him to a six-year, $126 million contract.
It also took Darvish time to mentally recover from the two losses. Coupled with arm issues that ended his 2018 season prematurely, his first season in Chicago wasn't exactly the best stretch of his career, and he's still wondering if what Houston did contributed.
"I'm better for what I went through," Darvish said. "But, yeah, everyone is wondering about pitching against them.
"It's tough to pitch. We're losing the strike zone. It's getting smaller. They want [us] to [pitch] quicker. And the hitters are stealing signs."
Cubs pitchers hope the playing field will be level after the league completes its investigations. MLB also is probing whether the Boston Red Sox used video to decode opponents' sign sequences and passed the information to their players in 2018, as alleged in a report by The Athletic.
Cubs players echoed what many others in the game have said: The Astros aren't the only ones cheating.
"You just don't know to what extent," right-hander Kyle Hendricks said. "Is it legal sign stealing or illegal? Hopefully, this offseason will eliminate that stuff."
Cubs president Theo Epstein praised the league's beefed-up investigation arm for its due diligence in the scandal. Both Epstein and star first baseman Anthony Rizzo are adamant that the Cubs have never gained an edge by using technology in that manner.
The goal is for no team to be able to do it.
"We applaud Major League Baseball's efforts to step up in this situation and make sure the games are played with integrity," Epstein said.