The Heisman Trophy winner suffered an apparent injury to his throwing shoulder in the fourth quarter of the Aggies' 45-41 loss to Auburn on Saturday. Manziel left the game and received treatment but returned to finish the game.
"I would list him as hopeful," Sumlin said Tuesday. "That's where he is right now."
Sumlin wouldn't go into details about the injury but did say that Manziel was wearing a sling as a precautionary measure during Monday's practice. He said Manziel will continue to receive treatment throughout the week.
Manziel injured his right shoulder when he was tackled by two Auburn defenders at the end of an 8-yard run with 14:36 remaining in the fourth quarter on Saturday. He grabbed his right shoulder before getting up and then fell to the turf as he was walking back to the sideline.
When testing his throwing arm on the sideline, Manziel could be seen wincing from the pain. Upon his return with 9:06 remaining, Manziel completed 9-of-10 passes for 102 yards and ran for a touchdown. He finished the day 28-of-38 passing for 454 yards and four touchdowns and 18 carries for 48 yards and a touchdown.
Texas A&M senior receiver Travis Labhart said Manziel was throwing the ball with his left hand at practice Monday.
Labhart said when he asked Manziel how he was feeling, the quarterback responded, "I'm feeling better."
Labhart said it meant a lot to him that Manziel tried to play through the pain.
"It makes me feel good to know that we have a quarterback that's back there that's doing whatever he can, putting off pain, to help his team win," Labhart said. "He gave us every opportunity."
"We feel confident in both of those guys," offensive coordinator Clarence McKinney said. "It's a different team when Johnny's out there and it's a different team when those guys are out there."
Sumlin said the rotation of quarterbacks at practice could change depending on how much Manziel is able to participate in the coming days.
Joeckel, who started the opener against Rice when Manziel was suspended for the first half, is 19-of-31 passing for 265 yards and a touchdown in three appearances this season -- including last week after Manziel was injured.
Hill, who made his debut against Sam Houston State the following week, is 6-of-11 for 83 yards and a score in two appearances.
McKinney, like Sumlin, said he's hopeful Manziel can suit up.
"Yeah, I'm hopeful," McKinney said. "We hope he goes. If he doesn't go, we can't cancel the game, so we've got to get another guy ready. But we're definitely hopeful that he'll play."