INDIANAPOLIS — The tight end leapt for an incompletion from Luck and was hit in the air by Raiders defensive back Tyvon Branch. His right leg landed awkwardly, and his hip hit the Lucas Oil Stadium turf.
Allen had hoped to be ready to play in last week's loss against the Miami Dolphins. But the hip did not respond well to treatment, and he went for a second opinion early this week.
Allen is the third starter lost to a season-ending injury by the Colts in the past six days, joining Ballard and left guard Donald Thomas. That fact might have strengthened the urgency to trade for Richardson.
"Like Donald, like Vick, like anybody we've lost at this point, it makes you sick," head coach Chuck Pagano said of Allen's injury before news of the trade broke.
Fleener, Dominique Jones and Jack Doyle are the other tight ends on the Colts' active roster, and seventh-round pick Justice Cunningham is an option on the practice squad.
Fleener's role is certain to expand, though Pagano did not get into many specifics. The 2012 second-round pick had four catches for 69 yards and one touchdown with Allen out against the Dolphins.
"I think we all saw why we took Coby where we took him, what he can do," Pagano said. "Last week, (he) played an excellent football game. Not only in the pass game, he blocked well. He'll continue to do that. He knows his role is obviously going to expand now. We got all the confidence in the world that he'll play at a high level for us, a winning level."
The Colts (1-1) start a difficult stretch of games Sunday with their first road game of the season against the defending NFC champion San Francisco 49ers (1-1). That game will be followed by a trip to division rival Jacksonville (0-2), then dates against Seattle (2-0), San Diego (1-1), Denver (2-0) and Houston (2-0).
Pagano said Indianapolis is well aware of the immediate challenge ahead.
"We're playing a really good football team," he said. "They're well-coached. They've got great players all over the place. So it's more about us. It's more about our preparation. It's more about our focus, knowing exactly what to do so we can go out there and play fast."