Castro's agent, Paul Kinzer, publicly announced Thursday that contract talks had been progressing, and both Castro and Cubs president of baseball operations Theo Epstein confirmed that a deal was close.
A report from ESPN on Friday said the two sides agreed to a seven-year, $60 million extension, but neither the Cubs nor Castro would confirm the report.
"I like [the numbers], but I don't know if it's sure yet," Castro said after the Cubs lost a 5-3 decision to the Reds in Game 1 of Saturday's doubleheader. "My agent, he'll let me know right away. He hasn't called me yet. I just let him handle it, and I just focus and play baseball. When it's done, I'm ready. I don't want to put pressure on myself. 'When it's finished, you let me know when it's finished.'"
The 22-year-old shortstop cannot become a free agent until after the 2016 season, and since the extension is longer than six years, the Cubs would be buying out his remaining arbitration years and at least two years of free agency.
In his third season with Chicago, the two-time All-Star entered Saturday's nightcap batting .276 with 12 home runs, 58 RBIs and 19 stolen bases. He led the National League with 207 hits last season, when he batted .307.
"I don't want to put pressure on myself," Castro said. "I don't want to think about money or contracts. I want to finish my season strong the last two months. I let him work; that's why he's my agent. He knows how to work contracts. I tell him, 'When you're ready, I'm ready too. If you're not ready, let me know and don't [let me] put pressure on myself.'
"I'll take it," Castro said of the reported dollar amount. "It's good for securing my life."
Mark Clements is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.