Reid slams fans’ misplaced cheers

Quarterback Jimmy Garoppolo received a warm ovation from the home crowd when he made his 49ers debut late in the fourth quarter of a 24-13 loss to Seattle on Sunday.

The problem: He entered to replace injured rookie C.J. Beathard, who was on his back and being examined by the medical staff as fans cheered.

Safety Eric Reid, who went onto the field to check on Beathard, was upset with the crowd.

“That pisses me off,” Reid said. “I’m disappointed in our fans. I understand the excitement about Jimmy, he’s a great player. But that’s not right. … People don’t understand what we go through as football players. And what our bodies go through. And he’s laying down on the ground and people are cheering. That’s messed up.”

Left tackle Joe Staley said the reaction was unfortunate, but not mean-spirited.

“I don’t think it was malicious by our fans and they were cheering his injury,” Staley said. “I think they just got excited, but, at the same time, you can’t do that. That kind of is a bad look.”

Beathard, who was hit in the left knee by defensive end Michael Bennett, said he wasn’t aware of the crowd.

“I was just focused on my knee and talking to the doctors,” Beathard said. “I wasn’t really focused on the fans or the crowd. So, no, I didn’t hear any of that.” New guys play plenty: After they were claimed off waivers a few days before kickoff, defensive end Cassius Marsh and defensive tackle Sheldon Day both played plenty in their debuts with the 49ers. Each had two tackles.

It helped that Day, who was claimed Monday, and Marsh, who joined the team Wednesday, had experience in the 4-3 defense the 49ers run.

Day, 23, spent his first two seasons in Jacksonville, and Marsh, 25, spent his first three seasons in Seattle. Both teams use a 4-3 scheme.

Said Marsh: “I just came here prepared to play. I wanted to play that much. I didn’t want to come off the field. I’m that type of player, so I’m thankful for my opportunities.”

Day and Marsh were part of a defensive front that limited Seattle to 90 rushing yards on 30 carries. More punishment: After he was rarely touched in a 31-21 over the Giants on Nov. 12, Beathard went back to being abused.

Beathard took three sacks and was hit 13 times Sunday. In his first five NFL starts, he’s taken 17 sacks and absorbed 52 hits. The 49ers faced the Seahawks without starting right tackle Trent Brown (shoulder), who was replaced by undrafted rookie Erik Magnuson.

“C.J. is a guy that lays it on the line for us every single week,” Staley said. “We’ve got to do a better job obviously . ... He’s tough. I know the kind of guy he is and the character he has. He’s a guy that we all kind of rally around.” Injury report: Running back Raheem Mostert, who has excelled this season as a specialteams gunner, left the game with a knee injury and did not return. He will have an MRI exam Monday.

Wide receiver Marquise Goodwin briefly left the game with a foot injury, and Staley missed one play after hyperextending his left knee.