In a race won by Honda's Davi Millsaps, Reed's nearest title challenger - Kevin Windham - managed third, leaving the Torco Fuels rider just 16 points behind Reed with three rounds to go in the newly classified FIM World Championship.
Millsaps turned in a strong, consistent ride to lead 18 of the 20 laps. Tim Ferry was second for Kawasaki.
"I found some really good lines in practice and got a good start in the main and took care of the rest," said Millsaps. "I got caught up in lappers but ended up fighting through. It was unfortunate what happened to Chad today but I felt like I rode a good race and really earned it tonight."
The big question will now be how quickly Reed, the dominant force for much of the season, can recover from his injuries - with the St Louis round taking place next Saturday.
"It will be interesting to see how Chad bounces back from his injury," said Windham. "It should be an interesting finish in the series. Today was really an emotional roller coaster for me. I came in here 27 points back and after his crash in the first practice it really pulled a 180 for me, and the season.
"Chad has got a lot of adversity in front of him right now. He has won a championship before so he knows what he's got to do to get it done. I've never won one and I'll be looking to do what I need to do to get my first."
In the AMA Supercross Lites main event, Ryan Villopoto grabbed the $1,000 Progressive Direct Holeshot Award to start the 15-laps up front with title leader Torco Trey Canard and Rockstar/Makita Suzuki's local star Nico Izzi second and third.
Villopoto, winner of the previous two main events leading into Detroit, quickly checked out and rode a near flawless race to earn his third victory of the season. Boost Yamaha's Ryan Morais finished second while Monster Energy/Pro Circuit/Kawasaki's Brendan Jesseman was third.
