CHICAGO -- The Indians' active roster looks a lot different on Tuesday than it did on Monday, to say the least.

Regulars Nick Swisher and Carlos Santana were placed on the disabled list, and the Indians called for George Kottaras and Jesus Aguilar to join the club from Triple-A Columbus.

Santana was placed on the seven-day disabled list with concussion-like symptoms, while Swisher will be lost for at least 15 days with a hyperextended left knee. Cleveland purchased Kottaras' contract from Columbus, while Aguilar was recalled from the affiliate for his second big league stint.

Swisher, who has been bothered by left knee soreness the past week, said he may have been overcompensating with his left leg because of right knee issues earlier in the season that have since cleared up. He started to limp after running out a ground ball in the sixth inning of Monday's game when he planted on first base with his left leg, and was removed two innings later.

"You know, I've been very fortunate in my career that this hasn't happened very often, so it's kind of uncharted waters a bit," said Swisher, whose only other trip to the DL came in 2005. "But I think just with where we are and how we feel, this is the best thing for the team and everything. Hey, just grind through it, get better these next couple of weeks, just get ready to come back and have a killer rest of the year."

Swisher said he was relieved when doctors told him Monday night there was no structural/ligament damage in his left knee. He'll travel back to Cleveland on Tuesday night for further evaluation on Wednesday. The diagnosis nonetheless turned out worse than what he first imagined after he was optimistic the knee would feel better Tuesday morning, but was "the complete opposite."

There's never a good time for an injury, but it is particularly difficult for Swisher, who said he easily gets cabin fever and was starting to turn around a subpar season. He's hitting just .211 in 49 games but had seven hits in last 22 at-bats (.318).

"I mean, just kind of seems like everything for me is kind of feeling better," Swisher said. "I feel like my stroke's a lot better, feel like my defense is back to where it needs to be, and then something like this happens. But it is part of the game. I'll just do my best to be the best cheerleader I can for these next couple weeks."

Santana, batting .159 for the season, began experiencing concussion-like symptoms after being struck in the head by a foul tip on Sunday against Baltimore. The backstop was a late scratch from Monday's lineup because he was feeling ill, and was sent back to the hotel.

"He takes a lot of foul tips anyway, but he took a pretty good one the other day," Indians manager Terry Francona said. He showed up [Monday and] his back was sore, which we knew. But when he started to run and get loose, he got a headache, so that's when you have to start to be cautious.

"We talked to their doctor and they couldn't call it a concussion yesterday, but he sort of seemed like he had the flu and he also had a temperature. But when you're having concussion-like symptoms, you have to err on the side of not playing. … We'll continue to evaluate him. If he gets to the point where he can increase his activity, we'll do that."

Kottaras was up with the Indians earlier this season, and he became the first player in franchise history to hit a home run in each of his first two plate appearances with the team. Kottaras played in 14 games for Columbus sandwiched around his stint with the Indians, and he's batting .119 in Triple-A.

The 31-year-old catcher elected to stick with the Indians when he cleared waivers following his first stint with the big league club.

"When you bring up guys for possibly a short stint, you want to make sure you a have guy that's professional and knows how to act in the clubhouse," Francona said. "That's George, and the fact that he hit a couple of home runs in the one game he played, we'll take that, too, but a guy that knows how to be a professional is very important, too."

Aguilar, 23, came up to the Indians earlier this month and collected his first big league hit on May 19 against the Tigers. He was hitting .286 with seven home runs for Columbus at the time of his promotion, and he ranked sixth in the Triple-A International League in OPS (.889).

The Indians also announced Tuesday that they designated right-handed reliever Blake Wood for assignment.

"This team is being tested right now, but that's one of the nice things about having depth," said second baseman Jason Kipnis, who will return to the starting lineup on Wednesday after being on the DL with a strained right oblique. "We have a lot of depth on this team. The guys we have in this locker room, is that we can throw in a different lineup there and still go out and compete and win games."