Sexson will go through a waiver period that expires on Monday and can be signed to another Major League organization for about $200,000 -- half the MLB minimum salary. The Mariners, meanwhile, are responsible for the remainder of Sexson's four-year, $64 million contract, which is slightly more than $7 million.
But management decided that having an unhappy player around would be worse than buying out the contract.
"The way I was going to be making up the lineup over the next couple of weeks, we were not going to be able to make Richie happy," Riggleman said. "He's been an everyday player his whole career and that wasn't going to happen here from this point on."
There have been reports of Sexson's impending departure for several weeks. But it appeared that he would remain with the team at least through the All-Star break.
That scenario changed on Wednesday afternoon.
After starting Sexson in seven of the first eight games this month, Riggleman put switch-hitter Jose Vidro at first base for Wednesday night's game against Oakland, which the Mariners won, 6-4.
Sexson's reaction apparently spoke volumes.
"He has been in the lineup every day, until yesterday, and when he wasn't, he was not happy about it," Riggleman said. "Nothing was said, but his body language indicated that he was a little perturbed by that.
"I thought, 'You know what? This is the way it's going to be in the near future.' So rather than have that, I felt it was time to do this. I wish that wasn't the case, but it wasn't going to happen here in the immediate future.
"This has been out there, hanging over everybody's head for a while, but we never really had specifically talked about when it might happen -- June, All-Star break, whatever," Riggleman added. "You know what? I was going to play Richie [on Thursday] and play him a game or two in Kansas City this weekend.
"But [not playing regularly] would create some negativism on the ballclub and we just can't have that."
Sexson's stint with the Mariners started with a bang.
He hit two home runs on Opening Day against the Twins in 2005, and went on to lead Seattle with 39 home runs and 121 RBIs, batting .263. He was an offensive force in '06, as well, batting .264 with 34 homers and 107 RBIs.
But he struggled mightily last season, batting .205 with 21 home runs and 63 RBIs, and again this season, hitting .218 with 11 home runs and 30 RBIs.
"This is something we have been contemplating for quite a while," interim GM Lee Pelekoudas said. "One of the things we committed to when we changed managers [on June 20] was letting these guys play under Jim for a while to see if the team improved as a whole, and see if players improved individually.
"We gave Richie a lot of rope to improve what he's doing, and there just wasn't enough improvement on his part. We wanted to get a clean break heading into the break, and decided this is the best time to do it."
First base will be manned by Vidro and Miguel Cairo this weekend against the Royals. The game plan coming out of the four-day All-Star break is still to be determined.
Left-handed-hitting Bryan LaHair, currently playing at Triple-A Tacoma, is an option, but he just returned to action after suffering a toe injury and he needs more playing time in the Minors.
Although catcher Kenji Johjima recently took ground balls at first base, there are no immediate plans to put him there in a game situation.
"That is not something I see happening in the near future," Riggleman said. "We want to take a good look at [Jeff] Clement behind the plate and if Joh starts coming on with the bat and forces himself into the lineup, I guess we would have to consider him over [at first base] some."
The Mariners also placed left-hander Erik Bedard on the 15-day disabled list on Thursday, retroactive to July 5. He will be eligible to return on July 20. To fill the two roster spots, Seattle promoted right-handed pitcher Jared Wells and infielder Tug Hulett from Tacoma.