The club believes that LaRue has a slight concussion following his involvement in Tuesday night's scuffle between the Cardinals and Reds.
"I do have a concussion, a slight concussion," LaRue said. "We'll see how we progress tomorrow and the next day."
LaRue also has several lacerations on his body and face and some soreness in his ribs.
"They're testing him for a concussion, and they're going to X-ray his ribs tomorrow," manager Tony La Russa said. "He's got a lot of soreness from where he was kicked in his ribs, plus all the cuts. They felt last night he had a concussion, so we'll be real careful with him."
Hill was chosen, rather than Triple-A catcher Bryan Anderson, in large part for logistical reasons. The Cardinals' Triple-A Memphis affiliate played a night game in Reno, Nev., on Tuesday, so it was not possible to get a player to Cincinnati in time for Wednesday's early start.
"It's a roller coaster," Hill said. "It's every kid's dream come true. This is where everybody works to be at, and I'm blessed to be here."
Suppan suffered a groin strain on Monday when he was running, a day after his scheduled start against the Marlins was rained out. The move is backdated to Aug. 1, so technically he could be reinstated to the roster within a week if he recovers quickly.
Hill's addition puts the Cardinals at 40 players on their 40-man roster.