Why? My guess is because you have more than 30 red packs.

Once you reach 30 red packs, prepare to have a player or players injured. You will keep getting injured until your red pack count is below 30.

Today I reached 31 red packs, I prepared myself for an injury during the league match. NO INJURY then. So the Champions' League match is next. 8th minute, after using only 5% condition, my MC gets injured, out for 4 days. Was I surprised? Definitely not.