University officials said it's an "unknown illness" and added Robinson could remain hospitalized the next few days while doctors continue to monitor him and diagnose the situation.
"I've been out there twice already to see him," said Arkansas coach Houston Nutt, who called the illness bizarre. "The first time, he was under a lot of medication and the second time he was sleeping.
"But the nurses and doctors said he's getting better. That's good news."
Linebackers coach Chris Vaughn said Robinson will be missed during Saturday's 6 p.m. game against New Mexico State in Reynolds Razorback Stadium.
"He's a great role player for our team," said Vaughn, who was headed to the hospital after Thursday's workout. "He helps a lot at linebacker, but especially on special teams."
Robinson's listed second on the depth chart at wolf (inside) linebacker behind Marcus Whitmore. Pierre Brown worked out with the second team at wolf during Thursday's practice.
Robinson made 11 tackles -- including two for a loss -- in 12 games last season. He also forced a fumble.
Robinson Remains Hospitalized
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