Ellsbury declined to be interviewed for this article, but General Manager Brian Cashman said that Ellsbury’s decline after the concussion was not a surprise and that it was explained by a neurologist last season.

“It’s almost like a period of time they’ve got to work through on their own, regardless,” Cashman said. “You have to grind it out and be patient.”

Eventually, Ellsbury found his peak form, hitting .337 with a .912 O.P.S. in September.

A tipping point in how baseball began to view concussions came 10 years ago with Ryan Church, a Mets outfielder whose career was derailed after he was allowed to continue to play for about a week despite experiencing symptoms from his second concussion in a two-month period. After a steep decline in performance, Church retired after the 2010 season at age 31.

His case spurred Major League Baseball to develop a concussion protocol, which was updated in the most recent collective bargaining agreement in 2016. It requires baseline testing of players when they enter professional baseball and dictates what tests must be administered before a player can return. It also allows for a seven-day concussion disabled list, removing an incentive for teams to keep players off what was then a 15-day disabled list. (It has since been shortened to 10 days.)

It also calls for a game to be stopped and a player to be evaluated on the field by a trainer if there is an incident that carries a high risk of concussion. If the trainer detects signs of a concussion, the player must be removed and examined further. But because a concussion’s cause can be so nebulous, it is hard to know just what constitutes a high-risk collision.

For example, Ellsbury was allowed to remain in a game last May after running into the wall at Yankee Stadium to make a catch. But when he was re-examined at the end of the inning, he was removed from the game. He missed a little more than a month.

It is not clear how Frazier became injured. He fell backward making a catch on Feb. 24 in Bradenton, Fla., and his head hit the wall. A trainer did not examine him until he came off the field at the end of the inning. Later in the game, Frazier caught a ball on the warning track and his shoulder hit the wall, jarring him. He did not begin experiencing symptoms — a headache and nausea — until after the team returned from Bradenton.