Cassel and his Chiefs are in the playoffs for the first time in five seasons.
"Tom is a guy that has been a mentor to me since the day I walked into the league," he said.
The NFL's top quarterback still fills that role even though they're separated by 1,300 miles and 28,886 career yards passing. They even spoke on Tuesday -- one of their frequent chats -- when Cassel wanted some tips about the Baltimore Ravens, Kansas City's opponent on Sunday and the team the New England Patriots beat 23-20 in overtime in their fifth game.
And what wisdom did Professor Brady impart?
"He told me everything we needed to do to beat them," Cassel said with a laugh. "He just kind of reiterated what we see on film, that they're a big, physical team. They've got an outstanding front seven."
Brady is eager to help, a celebrity superstar praised by teammates for his desire to be just one of the guys. After all, he said, Cassel is "kind of like a brother to me."
"I keep everything pretty generic," Brady said of their recent conversation. "I keep the secrets to myself. They're hard enough to learn."
The core of his curriculum is simple: do as the teacher does.
See how Brady watches film, the same one over and over again. Watch how he obsesses over his mechanics, keeping his left arm tight to his body as he delivers the pass. Pay attention to how he leads, barking at teammates who can take it and quietly correcting those who might be embarrassed.
"You try to lead by example and 'well-done' is always better than 'well-said,'" Brady said. "I always think you should go out there and do the right thing and you hope that the younger players see that and take a liking to it."
Cassel started watching closely in 2005, the year the Patriots drafted him in the seventh round out of Southern California where he never started while stuck behind Heisman Trophy winners Carson Palmer and Matt Leinart. His patience was tested again with the Patriots where he was a rarely used backup for three seasons.
In seven years, he had thrown just 72 passes.
Then, midway through the first quarter of the 2008 opener, Brady's season ended when he hurt his knee on a hit by Kansas City's Bernard Pollard. Cassel finally put his knowledge to use in a game and the Patriots won 17-10.
He started 15 games that year, but the Patriots missed the playoffs despite an 11-5 record. In late February 2008, he was traded to the Chiefs. He's started all but two games since, missing one this year when he had his appendix removed.
Cassel was very un-Brady-like last season, throwing 16 interceptions and 16 touchdowns while ranked 25th in passer rating in the NFL. He's been much better this season with 27 touchdown passes, seven interceptions and the eighth-best passer rating.
"Matt has more to do with it, certainly, than anybody," Brady said. "He's had an incredible year. We're great friends. I mean, anytime you spend as much time as we spend together, you develop just a great relationship."
But Cassel is coming off his worst game of the season -- 11 completions in 33 attempts for 115 yards, two interceptions, no touchdowns and five sacks. At least he could discuss his concerns this week with a three-time champion, two-time Super Bowl MVP and one-time regular-season MVP.
"Tom's been great. I continue to go to him for support," Cassel said. "He's a guy that I've constantly leaned on throughout my time here in Kansas City and throughout my career. He says go out there and have fun and enjoy the experience because there are a lot of other teams wishing they were in your position."
In the Patriots locker room, they sat a few feet apart. In practice, they worked on drills together. In meeting rooms, Brady prepared like a rookie doing everything simply to make the team.
"He was so meticulous, not just throughout the year but also when it came playoff time," Cassel said. "His awareness of what's going on, the game plan. We went over it three to four times the night before the game, so there wasn't any stone unturned."
Brady learned that from coach Bill Belichick, a stickler for detail, and passed it on.
"The biggest thing for any quarterback is just to watch him. Just do what he does," Belichick said. "He's relentless in his preparation, his quest for perfection. ... He does pretty much everything that you would want a quarterback to do."
Now Brian Hoyer is watching.
Like Brady and Cassel, he was lightly regarded coming out of college at Michigan State. The Patriots signed him as an undrafted free agent in 2009. Now he's in his second year as Brady's student.
"He loves the game and he has a lot of knowledge and I think he knows that he can share it even when he doesn't know he's doing it," Hoyer said. "I learn from him all the time, whether it's watching him out at practice or sitting in a room watching film with him and hearing things that he has to say.
"If you're going to be a backup quarterback, I think you'd want to be a backup to Tom."
Brady is just 33 and healthy. He's said he'd like to play into his 40s. So Hoyer's chance likely would come through a trade or an injury to Brady. Just like Cassel.
And now, with Brady's help, Cassel is in the playoffs.
"He is a guy that I constantly rely on, a guy I talk to every week, if not, every other week," Cassel said, "a guy I always try to emulate being a young player in the league. He had already won three Super Bowls and done everything you could possibly do. He is a guy also who humbled himself to help me.
"A lot of guys wouldn't do that, and he took the time out and I will always be appreciative."
Copyright 2011 by The Associated Press