Kinzer said the contract extension talks over the summer took a toll on the 22-year-old Castro, who signed a seven-year, $60 million deal on Aug. 28.
“The pressure got to him," Kinzer said Monday at the baseball winter meetings. “You should see his stats when we were negotiating. They were up before, then went down, then went back up."
Castro hit .264 in June, .235 in July and .252 in August after batting .317 through the first two months of the season. After signing the deal, he rebounded by hitting .324 in September.
“Now the pressure is off him," Kinzer said. “He can just go out and play baseball like he did his rookie season. Everything is behind him."
Castro also dealt with a sexual assault allegation during last offseason. Chicago police investigated the allegation but never filed charges.
In addition to the stress from contract talks, Castro's occasional mental lapses on the field and on the basepaths continued.
With all of that behind him, Kinzer said Castro should be ready to mature into the star the Cubs expect him to become.
“Sometimes you have to fail before you can really succeed," Kinzer said.
Castro, who bought his family a house after signing the contract, is debating playing winter ball, but Kinzer would rather he didn't.
“I just want him to go to the academy and train," Kinzer said. “But sometimes the lure of playing in front of family and friends is pretty great. I think he needs some rest."
Castro played in every game last season and all but four the year before. After becoming the youngest player in history to lead the National League in hits with 207 in 2011, Castro's overall numbers dipped in 2012. He batted .283 with 14 home runs and 78 RBIs and struck out 100 times for the first time in his career.