"It was my first number and it'll probably be my last," he said.
Don't think Haren is calling it quits now, but at age 34 and pitching his 13th season, this may be the end.
"I would say right now the chances are this will probably be it," Haren said. "I don't want to say this is it and pull a Brett Favre. That's why I said 'probably' [on Twitter]. At least I leave myself a little way out.
"Chances are this is it," he said. "After the season, I'll relax and see where I'm at. I definitely want to make a push to get to where this team wants to go."
The Cubs began play Sunday third in the Wild Card race behind the Pirates and Giants. Haren (7-7, 3.42 ERA) projects he has 10-11 starts remaining this season.
"All of them are going to be big starts and important for the team to keep pushing after what they've done this year," he said. "Coming here, the chemistry seems really good in the clubhouse. I won't mess up that. I just want to come in and do my job and give the team a chance to win every time out."
Fans should just ignore the radar gun when watching Haren's outings. After all, his Twitter handle is @Ithrow88 for a reason.
"I'm not the flame thrower I once was," he said. "I've gotten a lot smarter as a pitcher, I've gotten better at putting together scouting reports and sticking to them and being able to locate pitches. I've gotten better at that as I've gotten older. I'm going to come in and I expect to do the same thing I've been doing all year."
There are a few familiar faces. Miguel Montero and Haren were together in Arizona from 2008-2010, and the catcher has already invited the pitcher to stay with him in Chicago. Haren wasn't too sure about that.
"I like to sleep a little, too," Haren said, laughing.
He wanted to do some research on the Cubs' habit of batting the pitcher eighth in the lineup, although he isn't a bad hitter.
The Cubs had tried to acquire Haren in 2012 for Carlos Marmol but the deal was nixed because of questions about his medical records. Haren was on the disabled list that year because of back stiffness but said he never was sure why the trade didn't happen.
"I still don't know the particulars of what went down," he said.
He had some right shoulder stiffness in 2013, but has been healthy this season.
"I've had issues, but nothing that's kept me off the field," Haren said. "When you have as many innings as I've had and as much wear and tear, the medical report is going to be pretty long at this point in my career and it might scare some people. I'm here for two months, and I plan on making all my starts. I'll make the most of my time here."