"He's the first in the weight room and when he's done lifting he's on a treadmill running with a weighted vest on,'' Greenwood said during a recent telephone interview. "Man, he's a beast."
Last season the six-foot-five, 236-pound Johnson, nicknamed Megatron by a former teammate because of his huge hands, led the NFL with 122 catches for a league-record 1,964 receiving yards. He also set league marks for consecutive 100-yard games (eight) and consecutive contests with 10 or more catches (four) while tying Hall of Famer Michael Irvin's single-season record of 11 100-yard contests.
Greenwood and Johnson are teammates now after the Lions claimed Greenwood off waivers from the Kansas City Chiefs last Friday. Greenwood, 27, of Kingston, Ont., reached Detroit on Monday and has become entrenched in his new team's off-season program.
Greenwood has quickly sensed the Lions are using last year's dismal 4-12 record as incentive to return to the NFL playoffs in 2013.
"The work ethic I've seen on this team after just a week, they're on a mission," he said.
So, too, is Greenwood, eager for a new beginning in a new city after breaking into the NFL with Kansas City in 2010 as an undrafted free agent.
Greenwood capped a solid college career at Concordia in '09 by being named Canadian university football's top defensive player. The Toronto Argonauts took notice, selecting the six-foot-two, 235-pound linebacker in the first round, third overall, of the 2010 CFL draft.
But Greenwood signed with Kansas City, where he played in 48 games over three seasons, mostly on special teams. Greenwood accumulated 34 career tackles, including a team-high 15 special-teams tackles in 2011.
The Chiefs posted a league-worst 2-14 record last season, then made sweeping front-office changes that included the hiring of a new head coach (Andy Reid) and GM (John Dorsey).
Greenwood wasn't overly surprised when the Chiefs released him May 2. Fortunately, he wasn't unemployed long as Detroit made its waiver claim the next day.
"That's kind of the nature of the business," he said. "Being an undrafted free agent and sticking around for three years in one place . . . it was a pretty good run and I was lucky to stay there all this time.
"But change is good, too. I have a pretty good opportunity here to contribute on special teams and (former Detroit linebacker) Justin Durant has moved on (to Dallas) so there's an outside linebacker spot open for competition. It's pretty exciting and I'm looking forward to it."
Greenwood brings versatility to Detroit. Not only can he play linebacker and cover kicks but he was also a backup long-snapper with the Chiefs.
Greenwood experienced plenty of highs and lows in Kansas City. In 2010, the club finished atop the AFC West with a 10-6 record but the following season head coach Todd Haley was fired with the Chiefs at 5-8, replaced by defensive co-ordinator Romeo Crennel.
Kansas City's woeful 2012 campaign took a bizarre and tragic twist in December when linebacker Jovan Belcher shot and killed his girlfriend, then drove to to Arrowhead Stadium and committed suicide there. The shell-shocked Chiefs returned to the football field the next day, defeating the Carolina Panthers 27-21.
"Last year was probably the toughest of my career," Greenwood said. "It's something you can't even be prepared to deal with.
"We had the team meeting the Saturday night (hours after Belcher's suicide) and it was really hard to get up the next day and have to play a football game. Everyone was a professional about it and Jovan would've wanted us to play. We pretty much played for him but that was a very difficult situation to deal with."
Still, Greenwood will always fondly remember his time in Kansas City, especially playing at Arrowhead Stadium before the Chiefs' rabid fans.
"I loved the community, I loved the players, the whole organization is first class," he said. "In 2010 when we won the AFC West, it was a huge advantage playing at Arrowhead.
"It gets so loud there, it's right up there with how loud it gets in Seattle. Arrowhead is amazing and I'm so proud to have had the chance to play there."
However, being based in Kansas City made it hard for Greenwood's family and friends to see him play in person. That should be easier now, with Detroit's Ford Field being roughly a seven-hour drive from Greenwood's hometown.
"They're pretty ecstatic about that," Greenwood said.
Fortunately for Greenwood, he has the benefit of experience and football savvy as he tries to crack Detroit's roster. And there's also the lessons learned about how an unheralded rookie free agent and newcomer to American football can achieve the daunting task of making an NFL roster like he did in 2010 with the Chiefs.
"I went to Kansas City and played every play hard, took nothing for granted and gave it my all," he said. "I couldn't take the opportunity for granted and told myself I had to make the most of it and it worked out.
"But really all of that stuff is out of your control. All you can control is your play, just limit your mental mistakes, know your playbook and come to compete every day because there are guys there who are hungry and want your job. You can't take any days off (in NFL) because you're easily replaced."
Greenwood's arrival in the Motor City isn't good news for the Argos. But GM Jim Barker told The Canadian Press last week he'll continue patiently waiting for Greenwood.
"I want him to exhaust all of his NFL options and do what's best for him," Barker said. "When he comes here we want him to be committed and excited about being here and turn the page.
"We don't want him thinking, 'If I had just waited a little bit longer.'"
That's good, because Greenwood's focus remains squarely on continuing his pro career south of the border.
"I really haven't thought about it (CFL) too much," he said. "I guess it could be an insurance policy but my heart is down here right now.
"I'm going to see how long I can ride this wave in the NFL. Whenever it's over, it's over and I'll reassess my situation when I have to cross that road."
And with good reason. Greenwood said he needs to play three more NFL regular-season games to become a vested player and qualify to begin receiving monthly NFL pension cheques when he turns 55.
"I haven't thought much about it but it's something that would be awesome," Greenwood said. "I turn 28 in June and I feel like I'm just peaking.
"I've played three years of American football and learned so much as far as schemes and technique that I feel I'm not done yet, that's for sure. Football players want to play forever but there comes a day when the game passes you by and you have to let it go. Hopefully, though, that's a while down the road."