Kansas City Chiefs coach Andy Reid told the media Thursday the team was waiting for swelling to go down on Rashee Rice's knee before performing additional tests.
Three hours later, the Chiefs had enough information to know he'll be side-lined for at least one month.
The Chiefs placed Rice on injured reserve later Thursday, meaning he will out for a minimum of four weeks.
Rice was injured in last Sunday's 17-10 win over the Los Angeles Chargers, and it was reported he tore the anterior cruciate ligament in his right knee.
There has been no update on the extent of the injury, but a torn ACL would side-line him for the rest of the season.
The second-year receiver was injured on a bizarre play when he collided with Chiefs quarterback Patrick Mahomes, who inadvertently dove into Rice's knee as the two team-mates were attempting to tackle Chargers cornerback Kristian Fulton on an interception return.
Rice was carted off to the locker room shortly after leaving the field and eventually ruled out of the game.
His injury is the latest blow to a Kansas City team that is already without leading rusher Isiah Pacheco, who is recovering from a fractured fibula he sustained in Week 2.
It also leaves Mahomes without his top target, as Rice led the NFL with 24 receptions through this season's first three weeks.
No other Kansas City player had double-digit receptions entering Week 4, though Travis Kelce finally took on a larger role in the offence.
The veteran tight end had seven receptions for 89 yards against the Chargers after catching eight passes for 69 yards in the first three games.
Despite losing two of their top play-makers, the two-time defending Super Bowl champion Chiefs are 4-0 heading into their Week 5 Monday night game against the New Orleans Saints.