They're going to regret it.
Lewis, a New Orleans native, signed a five-year, $26 million contract with the Saints Thursday, according to the Pittsburgh Post-Gazette.
The Steelers didn't have to say goodbye, or in this case au revoir, to Lewis.
Lewis wanted to stay in Pittsburgh, according to Jim Wexell of Steel City Insider, and would have been willing to stay for $35 million over five years.
He took a lot less than that to go to New Orleans, and according to Spotrac , his deal with the Saints only counts $2.2 million against the salary cap next season.
Perhaps going home was a powerful enough lure for Lewis to take less money, but according to Spotrac the Steelers are about $13 million under the cap. They could have paid Lewis enough to cure his homesickness. They just didn't want to, and that became apparent when they reunited with William Gay on March 4.
The Steelers have earned the benefit of the doubt when it comes to personnel moves. They've been to three Super Bowls and won two since 2005.
But it's hard to agree with this one.
Lewis led the NFL last season with 28 passes defended, according to the Pittsburgh Tribune-Review. After the Steelers lost Ike Taylor for the season with an ankle fracture, Lewis showed some grit by playing hurt as the Steelers fought for a playoff berth. Lewis was a late bloomer with just one start in his first three seasons, but he's still just 26 and heading into his prime.
Yet the Steelers are smitten with Cortez Allen, who will take over Lewis' starting spot opposite Taylor while Gay becomes the nickelback.
Maybe the Steelers thought Lewis was a one-year wonder playing for a contract. So instead they're placing their trust in a one-month wonder.
Allen began to look like a starter when Taylor went down in Week 13 at Baltimore. He broke up three passes to help the Steelers beat the Ravens 23-20. The 24-year-old started the last two games of 2012, intercepting two passes and forcing three fumbles.
Even if Allen is ready to start, Lewis was a more proven starter.
The only weakness Lewis has shown is a lack of ball skills. He had no interceptions last season and just one in his career. He dropped a fourth-quarter interception at Tennessee that could have given the Steelers one more win last season.
Taylor's hands of stone have kept him out of the Pro Bowl. The Steelers might have been concerned about going through another season with two starting cornerbacks who can't catch the ball. After forcing 35 turnovers in 2010, the Steelers have forced that same number over the past two seasons combined.
Allen could be the ball hawk the Steelers defense needs, and Gay sealed two wins in 2011 with fourth-quarter interceptions.
If that's the Steelers' thinking, it assumes Taylor will be as good as new in 2013. He'll be 33 next season and coming off the most significant injury of his career.
Even if he's fully healthy, Taylor is getting to the age as a cornerback where his skills can decline at any time. What will the Steelers do if Taylor loses his effectiveness? Gay is best suited as a slot corner. He's not going to shut down a team's best receiver one-on-one.
When Gay returned to the Steelers, he said it was like being back with his "family," according the the Pittsburgh Post-Gazette.
Did the Pittsburgh Steelers make a mistake letting Keenan Lewis leave as a free agent?
That's a heart-warming story, but it would have been more heart-warming to know that the Steelers were set at cornerback for the foreseeable future.
Gay's cap figure is $1.16 million, according to Spotrac. That's about a million less than Lewis' cap hit with the Saints. Instead of bringing back Gay, the Steelers could have spent the money on Lewis. Now, unless Curtis Brown takes a big step forward, they could be looking for a new starting cornerback in a year or two.
Lewis might have dropped the ball in Tennessee, but the Steelers also dropped the ball by not keeping him. In the long run, that drop will be more costly.