At 6-foot-6 and 292 pounds, the defensive end is so big and showed such great burst that Fitzpatrick likened his new teammate to someone cooked up by a video gamer.
"He looks like a Madden character you create when you sit there and up-up the strength, and the guy keeps growing," Fitzpatrick said with a smile, referring to the popular NFL video game. "And then he's the fastest guy on the field, too."
As first impressions go, Williams made a big and immediate one among his new teammates in his practice debut.
Lining up on the left side, Williams showed off an array of moves that have made him one of the NFL's premier pass rushers, and led to Buffalo making him the league's highest-paid defensive player by signing him to a six-year, $100 million contract in free agency two months ago.
Williams showed power with a bull-rush that had tackle Chris Hairston back-pedaling into the pocket. And then there was Williams' speed, in which he twice burst past Hairston around the outside.
"I'm doing pretty good right now," Williams said. "To get a chance to come back out and run around with the guys and finally step foot back on the field, it was great."
Just don't ask him to make much out of participating in practice, even though this was Williams' first on-field team session of any kind in some eight months since a chest injury forced him to miss the final 11 games with Houston last season.
"Honestly," he said, "I don't think of anything else but going out and working."
More important, he shrugged off a scare that happened late in practice, when Williams knocked knees with fellow defensive end Chris Kelsay during a team drill. Flexing his left knee, Williams was held out of a few plays before returning to finish practice.
"I'll be fine," Williams said. "That's the last thing I'm worried about. Little things like that happen."
Williams is the centerpiece of a revamped defense that's making the switch back to the 4-3 system, and now being overseen by Dave Wannstedt, who took over as coordinator after George Edwards was fired after last season.
He joins a defense that allowed a franchise worst 5,938 yards, and struggled in pressuring opposing quarterbacks last season, managing just 29 sacks, 10 of which came in one game.
The Bills' pass rush could improve further if linebacker Shawne Merriman is able to recover from surgery he had to repair both his right Achilles tendon and shoulder last season.
Coach Chan Gailey said Merriman could be cleared to begin practicing as early as this week after the linebacker had a medical checkup on Tuesday.
They included quarterback Vince Young, who was signed earlier this month to compete with Tyler Thigpen for the backup spot. Young showed off a strong and accurate arm by completing a deep pass up the left sideline to Naaman Roosevelt.
Much of the buzz, however, focused on Mario Williams, and what his addition has meant in raising the hopes of a team that's attempting to snap a 12-year playoff drought - the NFL's longest active streak.
"Just the atmosphere in the facility: He brings that `it' factor," Kelsay said of Williams. "We'd be lying if we said we didn't expect a lot out of him. And I think he's just the guy to do it. He's a giant of a man. He's strong, he's fast, he's quick.
"Everything you expect out of the guy, he is it."
NOTES: Coach Gailey doesn't expect WR Stevie Johnson to be cleared for practice until the start of training camp. Johnson, who had groin surgery earlier this month, worked out on his own Tuesday. ... DT Kyle Williams (foot surgery), CB Terrence McGee (left knee) and RT Erik Pears (undisclosed) were among the veteran who also did not practice. ... The Bills close this week's voluntary minicamp with practices Thursday and Friday.