Sources said Jackson suffered a Grade 2 separation. A shoulder sprain is a tear of the ligaments in the joint.
Cornerback DeAngelo Hall also left the joint practice against the Houston Texans early with a groin injury, but it didn't appear to be serious. Gruden termed Hall day-to-day. A source told ESPN that Hall is expected to be sidelined for a week.
Jackson injured his right shoulder at the end of a one-on-one drill. He had caught a pass and hit the ground and then slid into a sled. He lay on the ground for a couple of minutes before walking off under his own power, grimacing. But he remained on the sideline with his right shoulder wrapped.
"It was the impact of when I hit the ground," Jackson said. "It's an unfortunate situation. I was just playing and trying to compete and just had a freak accident."
Jackson said the injury is not as bad as when he sprained his left shoulder in Week 2 last season, when he did not miss any time. One difference, he said, is that he hit the ground only; last season, a defensive back fell on his shoulder.
For now, though, it bothers him.
"It's very painful," Jackson said. "Being in camp now, I have time to heal it. I'll take the steps to better myself so when I get out here, I can be all right to play. ... It's hard to do certain movements, so I just need to strengthen it."
Hall tweaked his left groin and knee after slipping at the end of a play in which he was covering a Texans receiver.
He missed 15 games last season after tearing his left Achilles, but he said afterward that his Achilles was fine. He said he will be re-evaluated this afternoon.
Hall is the fourth Redskins corner to suffer some type of injury in camp. Bashaud Breeland will miss at least a month with a right knee injury, David Amerson is out with a sprained left shoulder, and Tevin Mitchel was waived Wednesday after suffering a season-ending torn labrum in his left shoulder.