The Milwaukee Brewers first baseman wrote in a since-deleted Instagram post he believes other teams have been guilty of similar infractions.
"I know from first hand accounts that the Yankees, Dodgers, Astros, and Red Sox all have used film to pick signs," he wrote, per NJ Advance Media's Brendan Kuty. "Just want you guys to know the truth. I personally think it's a tool in a tool belt to pick signs, but if we are going to be punishing people for it. Don't half ass it."
The Athletic's Ken Rosenthal and Evan Drellich reported in November the Astros had set up a monitor in center field at Minute Maid Park. Houston set up a television monitor between the clubhouse and dugout that broadcasted the feed in order to determine the meaning of the catcher's signs.
Upon completing an investigation, MLB suspended Astros manager AJ Hinch and general manager Jeff Luhnow for one year. Team owner Jim Crane subsequently fired both men.
Morrison listing the Red Sox and Yankees as possible guilty parties as well isn't surprising.
Red Sox manager Alex Cora was Houston's bench coach in 2017 and heavily implicated in the team's sign-stealing setup. Cora and the Red Sox are still under investigation by MLB.
Rosenthal and Drellich also reported Boston and its biggest rival engaged in tactics similar to those of the Astros:
"As far back as 2015, the Yankees used the video replay room to learn other teams' sign sequences, multiple sources told The Athletic. Other teams likely were doing the same. Sources said the Red Sox began doing it no later than 2016.
"Oftentimes it takes a player to show up and be like 'You f--king morons, you're not doing this?' said one American League executive."
The report noted the Red Sox and Yankees' approach came at a time before MLB laid down more explicit rules regarding the use of electronics to steal signs.
MLB issued fines to Boston and New York in September 2017 for attempting to electronically steal signs. Speaking at the owners' meetings in November, MLB Commissioner Rob Manfred told reporters how he was approaching sign-stealing with a different mindset after meting out those fines:
"I take myself seriously. I do. I think that when this began to bubble up during the '17 season, I looked backward at how the issue had been dealt with. … I wrote what I wrote because I did not believe that the disciplines that had been handed out in the past were in line with the significance of the issues that we were dealing with. So I do view [the public statement] as a line of demarcation."
According to Jeff Passan of ESPN, Cora's punishment is expected to be "harsh" when the ongoing investigation into the Red Sox concludes.
The Dodgers have so far avoided the kind of claims swirling around the Astros and Red Sox, who beat them in back-to-back World Series.