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Colts rookie quarterback Anthony Richardson left Sunday's game against the Titans with a right shoulder injury, marking the third time in five weeks the first-round draft pick has been forced to the sidelines. 

Richardson suffered an AC joint injury, the Colts announced Monday. NFL Media reported more details, saying it's a Grade 3 sprain and that he could miss at least a month and possibly longer. Colts head coach Shane Steichen did not offer much on how long Richardson might be sidelined, only saying the signal-caller will miss "some time," via CBS Sports Lead NFL Insider Jonathan Jones.

NFL Media reported the rookie was diagnosed with a Grade 3 AC joint sprain and could miss at least a month and possibly longer. While Steichen did not say how much time Richardson would miss, IR remains a possibility.

Richardson received an MRI at an imaging center, according to CBS Sports HQ Senior NFL Insider Josina Anderson. Richardson, who wasn't hospitalized, will seek a second opinion regardless of the results of his initial scan, per Anderson. 

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Richardson stayed on the ground after falling on his right shoulder while being tackled by Tennessee's Harold Landry in the second quarter. He was then escorted to the locker room for further evaluation, with backup Gardner Minshew taking over under center. 

The Colts officially ruled Richardson questionable to return after his exit, then downgraded him to out early in the second half. 

Drafted fourth overall this spring, the Florida product previously sat out Week 3 with a concussion suffered the week prior. He also briefly left the Colts' season opener with an apparent lower-leg injury suffered late in Indianapolis' loss to the Jaguars.

Minshew filled in as the team's starting QB against the Ravens, guiding Indy to an upset victory. He helped the Colts carry a tight lead into halftime against the rival Titans on Sunday, while Richardson left having gone 9 of 12 for 98 yards through the air.