Originally, injuries were never shown while playing a match, to prevent "spoiling" the outcome (since in reality, matches are actually simulated within the 15 minute window before the match starts when you can't change your lineup any more).
Some more enterprising managers would abuse that fact to intentionally transfer list high salary injured players so that their auctions would end during a match, to try to catch someone off guard into bidding more than they would have if they knew the player was injured. So they changed the system so that the player's page will always show the player's injury, even during a match. But the team roster page still hides injuries during match and they didn't really fix the "Injuries are not shown while playing a match" message to only show on that team roster page and not the player page. (And it's been that way for more than 50 seasons, so I wouldn't bet money that they'll ever fix it
)