As they slid toward the SAFER Barrier on the outside wall, Wickens' vehicle launched over the front of Hunter-Reay's auto and into the catch fencing, where it spun violently before landing on the track. The nose of the vehicle hit the outside wall as the auto traveled at full speed.
"It is just bad, but I am so glad he is awake and alert".
Rossi led 180 of 200 laps to win his second straight race and third of the season, slicing Dixon's lead to 29 points with three races left.
The race was red flagged and drivers were allowed out of their cars.
Wickens was "awake and alert", but has been taken to a local hospital for further evaluation. Never good to see a vehicle go up like that, but I know he is in good hands.
The rest of the race is now delayed, but it's expected to resume as soon as debris is cleared from the track.
"All we can hope for is that everybody is going to be OK", points leader Scott Dixon said.
Pocono Raceway president Ben May said about 80 feet (24.4m) of fence and a few posts were damaged in the wreck, repairs Bourdais described as "pretty lousy" before he got back in the vehicle.
The collision came as they exited turn two and caused Hunter-Reay's auto to careen into the wall, and Wickens' vehicle was pulled along with it.
In 2015 at Pocono, Justin Wilson died from a head injury when a piece of debris from a crashed auto bounced off the track and hit his helmet.
James Hinchcliffe, Takuma Sato and Pietro Fittipaldi were also involved in the crash and multiple drivers were injured.
It was the second crash of the race, as Rahal wrecked Spencer Pigot while the green flag flew on Lap 1. Pigot spun and hit the wall. He left the vehicle and was out of the race.
Rahal got a penalty for avoidable contact and the first six laps were run under caution.
Greenwood performed the "The Star Spangled Banner" and his signature song "God Bless the U.S.A." before the race the day after he did the same at the NASCAR race in Bristol, Tennessee. But Greenwood went out on a high note and nailed both.
Pocono is one of the fastest tracks on the IndyCar calendar. The 38-year-old New Zealander is seeking his fifth Verizon IndyCar Series championship.
He not only has to hold off Rossi, but former series champions Josef Newgarden and Power aren't out of contention yet.
Dixon, who had only qualified 13th, had to weave his way through the debris.