SAN DIEGO -- The struggling Reds have been beset with a lot of injuries already in this young season, but they felt the latest one -- a concussion for catcher Tyler Stephenson on Tuesday night -- carried an added insult that some of his teammates did not appreciate.  

During the Padres’ 6-2 victory at Petco Park, which extended Cincinnati’s losing streak to eight games, Stephenson left the game in the bottom of the first inning following a hard collision at the plate with baserunner Luke Voit.

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“He’s doing fine, but obviously a concussion is concerning all of the time, anytime that happens,” Reds manager David Bell said. “It’s scary anytime there’s any sort of head injury. I spent a couple of innings coming up and checking on him. He seemed OK. He’s going to be fine. Obviously, we’re concerned.”

Stephenson was placed in concussion protocol, which means he will be out for a minimum of seven days.

Afterward, Voit expressed well wishes for Stephenson, while noting that he was simply trying to avoid the Reds’ catcher at the plate.

“It was going to be, obviously, a close play,” Voit said. “You, obviously, have no idea where the ball's going to be. It just happens, man. You can't really explain that situation. I try to avoid, try to be safe. That play ended up happening. I think it was just impressive he hung onto the ball, honestly.”

The Reds, to say the least, did not view the play quite so innocently.

“I don't like it all,” said Reds left fielder Tommy Pham, who hit his second homer in two nights with a solo drive in the top of the first inning. “They can say what they want, everybody on that side, they know I get down. I know a place here. I know an owner who will let me use his gym if we need to settle anything. … That play was dirty.”

Two batters into the bottom of the first inning, San Diego took a 2-1 lead on Manny Machado’s two-run homer against Reiver Sanmartin. With one out and Voit on first base after a walk, Jurickson Profar lined a double to the left-field corner. Voit tried to score while Pham retrieved the ball and made a good relay throw to shortstop Kyle Farmer.

Farmer fired a strike to Stephenson at the plate, but the throw was slightly up the line. As he slid, Voit’s arms went high and then came down as he landed hard on Stephenson’s head. The catcher held on to the ball to make the tag. As Voit was called out, Stephenson immediately looked dazed and remained flat on his back.

“Looking back at the replay, I’m not too happy about the slide,” Farmer said. “Not too often you see a runner slide and grab someone’s head and slam it to the ground like that. You’re usually trying to reach for the bag. Maybe that’s what he was trying to do. After looking at the replay, it looked like a wrestling move to Tyler’s head and snapped it down.

“I’ve caught before, and it’s a scary play, especially when you have a big guy like Voit coming down. I’ve never really seen someone’s hands go to a catcher’s head on a slide.”

Voit is listed at 6-foot-3 and 255 pounds. Stephenson is 6-foot-4 and 225 pounds.

“If Luke wants to settle it, I get down really well,” Pham said. “Anything. Muay Thai, whatever. Like I said, I've got an owner here who will let me use his facility.”

Stephenson spent several moments on his back while being examined by athletic trainer Tomas Vera as Bell and concerned teammates looked on. The 25-year-old was helped up and was able to walk off the field with Vera.

As that was happening, Bell used a replay challenge to see if Voit violated the rules on initiating a collision with a catcher. After a review, it was determined no rules were broken on the slide.

“I didn’t have a problem with the slide,” Bell said. “But it’s a helpless feeling when one of your players gets hit in the head and they’re lying there. It’s scary for me. I may have just reacted and it felt like the only thing I could do. It didn’t end up hurting us because we didn’t need the challenge. I thought it could’ve been an illegal slide. That doesn’t mean that Voit did it intentionally.”

For his part, Voit, a former catcher himself, noted that he did not have any intention of trying to injure Stephenson.

“I wasn't trying to take him out or anything,” Voit said. “I guess my elbow just kind of smoked his head a little bit. Hope he's all right. No hard feelings.

“It's baseball. I wasn't trying to make it dirty or anything. I'm just trying to make a play obviously. Unfortunately I was out. So I ended up getting the short end of the stick on it, too.”

Making the situation even more difficult for Cincinnati was that backup catcher Aramis Garcia was already in the lineup as the designated hitter. Since Garcia replaced Stephenson at catcher, rules stipulated that the Reds lost their DH for the remainder of the game. Sanmartin had to hit for himself in the second and fifth innings.

“We didn’t prepare ourselves for that,” Sanmartin said through translator Jorge Merlos. “Sure enough, right after I was done with that inning, I had to go take an at-bat. It was out of nowhere for me, but I guess it kind of felt normal afterward when I got in the box.”

The Reds already led the Major Leagues with 12 players on the injured list before Stephenson was hurt. Taxi squad catcher Mark Kolozsvary is the expected replacement on the active roster. Kolozsvary, who does not have a big league at-bat, was summoned from Double-A Chattanooga on Saturday.

Cincinnati and San Diego will complete their three-game series on Wednesday afternoon. The two teams are scheduled to meet again at Great American Ball Park next week, with Tuesday’s play likely to remain fresh on the minds of both clubs.

MLB.com reporter AJ Cassavell contributed to this story.