Jesse Hahn - Jesse Hahn had to go under the knife late in the 2018 season, and is expected to miss a few months moving into 2019. The Royals depth at the pitcher position is a little thin, and even as a long relief guy, Hahn would be needed. Hahn has shown some potential at times, and a move to Kansas City isn’t the worst place to go for a struggling pitcher. He is a big righty that would be needed in 2019 regardless of his inconsistencies.
Salvador Perez - Salvador Perez struggled at the plate last season, and it has a lot to do with his thumb injury he battled and missed time with over the season. Perez had thumb surgery right after the season was finished, and is expected to be back for 2019. If Perez has any lingering issues, we saw the effect it had on him at the plate. Perez has been very durable in his career, so it would be a shock to see Perez miss a ton of time.
Danny Duffy - As mentioned with Hahn, the Royals pitching staff is very thin. Danny Duffy has gone through struggles, but at times can be a strong left-handed arm. The Royals will be looking to lean on him if healthy to lead the staff, but the bigger concern is if the arm issues continue, they have more than $40 million to pay him over the next few seasons.
Coco Crisp had major injury for the Kansas City Royals back in 2009, where he tore his rotator cuff in the middle of the season. This came right after Alex Gordon had a tear in his right hip, causing him to miss some time. This left their outfield in shambles that year. Many forget that back in 2012 Salvador Perez had to have surgery on his knee to repair a torn meniscus.
In that same season, Joakim Soria had to have Tommy John, which left a hole in their bullpen. Danny Duffy also went down with a major UCL injury in his pitching elbow, which eventually required Tommy John.
Back in 2016, Mike Moustakas had a torn ACL, which caused him to miss the whole season. Kris Medlen was right there to follow him, with a right rotator cuff injury. It was the start of pitching injuries for the Royals, as Nate Karns had nerve irritation in his elbow and Matt Strahm tore his patellar tendon in his left knee. Jesse Hahn later followed up with a sprained UCL, causing him to miss the entire 2018 season.