CHICAGO -- In his second tour with the Cubs, Yu Darvish found a comfort level with both his surroundings and his teammates. He moved beyond the injuries that plagued him two seasons ago, entertained reporters with one-liners and dominated down the stretch on the mound.

All of that together helped create a situation that Darvish did not want to leave. On Thursday, the first official day of baseball's offseason, a source told MLB.com that the pitcher will not opt out of his contract with the Cubs. Darvish has $81 million remaining over the next four seasons as part of the six-year deal he signed with Chicago prior to the '18 campaign. The Cubs have not confirmed Darvish's decision.

At the end of last season, Darvish described the Cubs as the "perfect" environment for both him and his family.

"My kids and my wife love Chicago, especially the kids," Darvish said. "They only care about the Cubbies."

During that gathering with reporters in St. Louis shortly before the Cubs' season concluded, Darvish said he did not think he would opt out of his contract. Now, it's official, giving Chicago the chance to proceed with its offseason plans knowing they have an arm with ace potential in the rotation. Adding starting depth is already a winter need for the Cubs, so retaining Darvish was important.

As things currently stand, Darvish will be joined in the rotation by Jon Lester and Kyle Hendricks. The Cubs will likely pick up the $10.5 million team option for José Quintana, but the team has until Monday to make that decision. Veteran lefty Cole Hamels is eligible for free agency, leaving a vacancy in the big league rotation. Two in-house depth arms include Adbert Alzolay and Alec Mills.

"It's an accomplished, an experienced group," Cubs president of baseball operations Theo Epstein said at the end of the season. "But with the experience, we could stand to add some younger talent and refresh the group as well. We certainly need to add depth and we need to add some youth and a little bit of a different look to the staff as well as we look forward."

Overall, the 33-year-old Darvish went 6-8 with a 3.98 ERA in 31 starts last season, piling up 229 strikeouts against 56 walks in 178 2/3 innings. It was a solid comeback showing from '18, when he posted a 4.95 ERA in eight starts during an injury-marred opening act with the North Siders.

Darvish's final '19 line does not tell the whole story of his season, though. The righty had a 5.01 ERA in the first half with the fourth-highest walk rate (11.7 percent) in the Majors. In the second half, Darvish fashioned a 2.76 ERA and had 118 strikeouts compared to seven walks in 81 2/3 innings. His 2.2 percent walk rate during that stretch was the lowest among qualified Major League pitchers.

"He deserves all the credit in the world. It's been a really impressive turnaround," Epstein said in August. "This has been a really extreme turnaround, specifically, with his control. I think it just goes to show the importance of confidence and aggressiveness and comfort. He never backed down."

In the second half, only Houston aces Gerrit Cole (39 percent) and Justin Verlander (36.6 percent) had a better strikeout-minus-walk rate than Darvish (35.6 percent). Darvish's 37.8 percent strikeout rate after the All-Star break also led the National League, trailing only Cole (44.1 percent) and Verlander (40.7 percent) in MLB.

What did Darvish think sparked his dramatic in-season improvement?

"I have a lot of reasons -- like 20 or something," Darvish said. "But, No. 1, because of the Cubs, you know? The organization, like I've told you, not only [the manager and front office], a lot of guys were patient and they always helped me. That's what helped me."

And, ultimately, it helped Darvish decide to stay with the Cubs.

"This organization is perfect for me," Darvish said. "I like Chicago, the city. And the people are nice. And this organization lets me do whatever I want, so I'm so comfortable."