Seymour was acquired last Sunday for a 2011 first-round pick but did not report to the Raiders, raising speculation he did not want to join Oakland.
In his first public comments since the deal, Seymour told the Boston Herald on Saturday that he was taken aback when coach Bill Belichick told him of the trade, which is sending him from one of the NFL's model franchises to the one with the worst record over the last six years.
"First of all, I was blindsided by this whole event," Seymour told the Herald. "When you get blindsided, you should take a moment to gather your thoughts. I have a lot of personal issues more pressing than football."
Seymour has four children and is the guardian of a 15-year-old cousin, who had recently joined him in the Boston area. Seymour's family will return to South Carolina, while he flies to Oakland on Saturday to join his new team.
Seymour said he has been in regular contact with Raiders owner Al Davis and coach Tom Cable. He said he has talked with the Raiders about a possible contract extension or a promise not to be designated the franchise player following the season, but was given no guarantees.
Seymour turns 30 next month and is in the final year of a contract that pays him about $3.7 million this year. Had he not reported, he could have been suspended for the season and had his contract roll over to 2010.
"I didn't demand anything to go there," he said. "I'm happy to go there, and I told them that. But me and my family have never been separated. This was a difficult transition."
Seymour has been a stalwart in New England since being drafted sixth overall in 2001. He played on three Super Bowl winners with the Patriots, recording 39 career sacks and being selected to the Pro Bowl for five straight seasons beginning in 2002.
The Raiders were looking to upgrade a run defense that was the worst in the league during the exhibition season and has been the worst over the past six seasons. Since going to the Super Bowl following the 2002 season, Oakland has had the worst run defense in the NFL, allowing 141.7 yards per game on the ground and 122 touchdowns rushing.
Despite his late arrival, Seymour said he plans to be on the field for Oakland's opener Monday night against San Diego.
"I'll be the guy on top of the quarterback," he said.
Cable is expected to address Seymour's pending arrival after practice Saturday.