Teammates for nine seasons in San Diego (1994-2002), the two veteran defenders have been reunited in New England.
Harrison, a strong safety, won back-to-back Super Bowls after signing with the Patriots in 2003. Seau, a 12-time Pro Bowl linebacker, joined the team last Friday, just four days after announcing his retirement.
"Junior was a guy who taught me how to be a pro," Harrison said. "I really tried to model my work ethic after Junior's. Of course, when I won that (first) Super Bowl, I was thinking, I wish my man Seau was here to experience this.
"Guess what? He's here. Now that's going to put that much more (pressure) on us (as a team) to take it to that next level. Because you love to see a guy like Corey Dillon win a Super Bowl, as well as a guy like Junior Seau because he meant so much to the game for so many years."
Seau, 37, practiced for the first time with the Patriots yesterday. Before hitting the field, he said he didn't expect to make an immediate impact.
"I just came from surfing a week ago," he said. "I haven't hit anyone. This is going to be the first day, a tester, the start of a new year. I'm going to take it day by day and go from there.
"I'm trying to be a student of the game. I'm here early in the morning. I'm just going to throw my nose into the playbook and absorb everything I can."
Harrison and Seau played in one Super Bowl together, after leading the 1994 Chargers to an AFC championship. San Diego lost Super Bowl XXIX, 49-26, to the San Francisco 49ers.
Harrison said he thinks Seau has a lot to offer the Patriots, whose linebacking corps is in flux. Tedy Bruschi is out for at least the rest of the preseason with a broken wrist. Monty Beisel, the other projected starter inside, also has missed time with an undisclosed injury. Beisel returned to practice yesterday.
"I know what Junior brings to the table," Harrison said. "I know that he's a leader. I know that he's an energetic guy. He's a very unselfish player. And I know that he can still play."
Patriots coach Bill Belichick said Seau would play inside linebacker. "How he fits in, I don't know," Belichick said. "It'll depend on how he does."