One of the biggest worries about Salvador Perez in regards to his future is overuse. Ned Yost and the Royals have used Perez a lot in his young-ish career. Since 1913 only seven other catchers have played as many games as Perez through his current age.
Bench is clearly at the top and Rodriguez has a decent lead on Wynegar, but the last four or five are all pretty close to each other.
Another thing worth mentioning about Perez is that he's big for not only a catcher, but for a baseball player in general. Baseball Reference lists Perez at 6'3" and 240 lbs. Looking at catchers who are at least 75 inches tall and weigh at least 225 pounds, Perez stands far above all else in most games played at catcher (again through age 26).
This was one of the main concerns when the Royals renegotiated Perez's deal, giving him much more money. Bigger baseball players (and athletes/humans in general) generally don't age quite as well as their smaller companions. I'm not tall nor particularly old but I'm not aging very well myself. I can't imagine being 5-6 inches taller and 50-60 pounds heavier, crouching for a few hours a day while taking 2-3 swings every day (yes, that's a riff at Perez's plate discipline).
So maybe the next chart won't be much of a surprise to you when you remember Perez's size, workload, and the stresses of catching:
That's a 20-game rolling average of Perez's wRC+ over this season. You can see to start the year Perez was pretty hot, cooled off considerably in early-mid May before going on a tear from late-May to early July. However from July on Perez has been struggling.
Perez was never never one to walk, but they have been halved in the latter frame of the season. He's hitting for far less power too. There is some perhaps BABIP luck attributing the poor second half numbers but that could also be a cause of poor hitting. After all, he's hitting 55% worse since play resumed from the break.
Leading up to the All-Star break, Perez was on pace to putting up the best season of his career, heavily rewarding the Royals faith in paying him more money.
He was doing everything. He had the most home runs among catchers, he was hitting for power, and playing the best defense. Then the 2nd half started...
He's been surpassed in home runs by many of his catching counterparts, and is hitting far worse than them too. The defense is still there but it's not good enough to make up for his offense, leaving Perez basically replacement level this half.
Now Perez hasn't had the worst drop off from the first half to the second but he's been among the ten worst hitters during the back stretch. He's joined in his decline by teammate Eric Hosmer as well, likely being partially responsible for the horrid July the Royals had.
Perez has never been patient but there was a time early on in the season when he showed some semblance of patience. But as the season went on, Perez has returned back to his old ways of swinging at outside pitches and swinging more overall.
This isn't new for Perez. He's slumped in the second half before too.
For the first half of each season of his career, Perez owns a 106 wRC+. In the second half of each season, he has an 88 wRC+ (18% worse).
Maybe it's overuse. Perez has played a lot of games in his career and Ned Yost hasn't stepped off the gas. In the 2nd half so far this season, Perez has played in more games than any other catcher.