Opening day is supposed to be a time when smiles come easy, when managers and players look to the long and daunting grind ahead with a profound sense of hope and optimism.

Royals skipper Ned Yost wasn't having any of it.

He'd lost All-Star catcher Salvador Perez for up to six weeks because of a freak knee injury. One of the key pieces in his bullpen, Nate Karns, is headed to the disabled list with elbow inflammation. And two of the club's top prospects expected to help sooner than later, infielder Adalberto Mondesi and outfielder Bubba Starling, are likewise headed to the disabled list.

The Royals were also forced to designate for assignment erstwhile top prospect Kyle Zimmer, a hard-throwing pitcher whose troubling injury history has prevented him from realizing his potential.

Throw in the fact that Yost prefers to get into the day-to-day grind, rather than sit through the pomp and circumstance of opening day, and he was in a particular grim mood Thursday.

"You kind of want to get into it, get into a routine," he said. "Opening days are fun, especially your own opening day. Road opening days aren't so fun. But you get through it and get to the point where you start playing every day. I'm looking forward to that."

He was probably looking forward to it a bit more a few days ago.

Perez sustained a Grade 2 sprain of the medial collateral ligament in his left knee on Tuesday, when he was carrying a suitcase up some stairs in his home. Perez heard a pop and called trainer Nick Kenney, who in turn called Royals general manager Dayton Moore.

Moore made Kenney deliver the news twice. He didn't believe it at first.

"Mentally when you enter an off day or come out of an off day," Moore said, "you know the general health of your team. So Salvy was in great health, and he had an excellent spring on all fronts, so I didn't see this coming at all."

The five-time All-Star will not require surgery, but he will be sidelined at least a month and likely longer. That means Drew Butera and Cam Gallagher will share catching duties beginning with the Royals' opener against the Chicago White on Thursday at Kauffman Stadium.

Karns, a right-hander who hoped to earn a spot in the rotation, was instead headed for a crucial role in the bullpen. But stiffness in his elbow during his last two starts of spring training forced him to the DL with elbow inflammation, leaving the Royals' pitching staff in a bit of flux.

"Entering spring training he was probably going to be a reliever for us, the vein of Mike Minor. Get to the 60- or 65-pitch mark and start to fatigue," Yost said. "We'll get him completely feeling good and when he comes back he'll go back to the 'pen."

Mondesi is a slick-fielding infielder who spent time with the big league club last season, while Starling is a former first-round pick whose defense has masked his offensive shortcomings.

The open roster spots were filled by right-hander Blaine Boyer and infielder Ryan Goins, who had their contracts selected, and Gallagher, who was recalled from Triple-A Omaha.

The Royals also designated for assignment veteran pitcher Wily Peralta, though it was Zimmer's name that raise eyebrows. The club has long hoped that the fifth overall pick in the 2012 draft would stay healthy long enough to contend for a spot in the big league roster.

Instead, he's barely been healthy enough to find his way into a minor league rotation.

Moore said he's hopeful that nobody will claim Zimmer, and the club can ultimately spend more time working on his rehab. But even if that happens, nobody is quite sure what his future holds.

"He just simply ran out of time. That's one thing every player experiences, the clock is always running. There is always another young player on the horizon," Moore said. "We weren't in a position to stay longer with him while he was on the roster. Now we hope to keep him in the organization."