The 21-year-old collided with Helm during a drill in the camp's first 10 minutes at Traverse City's Centre Ice Arena, resulting in both going to the hospital.
"I heard it, and turned around and saw them go down," said Johan Franzen, who missed much of last season after a concussion. "It was really scary. The kid was in so much pain. I was sitting down on a knee and kind of feeling a little sick for a while."
Verrier suffered a broken right leg and Helm will go through the league's concussion protocol, said Red Wings coach Jeff Blashill, who replaced Mike Babcock, now with Toronto.
"I just heard it," forward Tomas Tatar said. "I turned, and two guys are on the ice. For sure, you don't want to see this happen on the first drill of camp."
Tatar said they hit when they weren't looking. Verrier was carted off on a stretcher. Helm was able to leave the ice under his own power but didn't return to practice.
Richards figures to replace Stephen Weiss as the second-line center. The two-time Stanley Cup champion and former Conn Smythe Trophy winner is coming off a 12-goal, 37-point season with the Chicago Blackhawks and signed a one-year contract with Detroit in July.
He's currently slated to be on a line with Helm and Tatar, who had a breakout 29-goal season in 2014-15.
"He's an experienced guy," Tatar said. "He won a Cup last year. ... Hopefully the chemistry will get going."
A 14-year veteran, Richards has put up 904 points in 1,058 career NHL games.
"It was good to get the uniform on for real," Richards said. "No matter how many years you do it, there's always butterflies the first day. I like being able to meet the guys. For me, it's more about that."
Green adds a much-needed right-handed shot to a defense corps that featured all lefties last year. Franzen said that Green will especially aid the power play.
"It's going to help," Franzen said. "He's got good hands, and that's probably what he's best at."
Pavel Datsyuk did not participate in practice as he recovers from ankle surgery. Veteran forward Daniel Cleary also missed the first day of camp with a leg injury. ... Tyler Bertuzzi and Dylan Larkin scored goals for Team Howe in the day's scrimmage, with Gustav Nyquist and Justin Abdelkader striking for Team Lindsay.