You dont play your best players in a highly competitive game against your biggest rivals when they arent match fit, doing that is highly likely to get them injured.
Would you rather play these ”unfit players” in the Premier League match? You think that won’t be a highly competitive game? Why would the risk of injury not be present in that match?
What about the Rosenborg match? Using your logic we couldn’t have played anyone as they were not match fit.
Anyway as I said we have a squad. 18 players, easy. Some of them are not able to play 90 minutes and that’s what you have six subs for.