I also want to ask the same question, because as a football lover, a game where both players are equally strong will be much more exciting, the ideal composition of each team in battle will not be saturated in giving each other the best attack and solid defense. If both teams are not strengthened by superior players, it will not be very exciting and perhaps the coach will be considered careless because he wasted the opportunity, where the UCL is proof of the strength of the teams from each league. It's so strange to have to play with such a messy composition, I'm sure that won't happen. No matter how bad the club condition is when they enter the UCL, they will show their best performance compared to their performance in the Domestic League.
As football fans, of course we want to see the performances of the best players, which makes competition tight and most of the best players are in big clubs. If these big clubs play without their best players, this will certainly make it difficult for the club itself because its performance will decline and for Do we see a game without the best players when they are fighting each other to the next round? Isn't that too risky, as fans we definitely don't want to see our favorite club, one of the players gets injured because he will be absent and won't be able to help the club. Especially if he is a the coach's mainstay players will certainly make the coach feel frustrated having to rack his brains to rotate his players and create effective strategies.
Big clubs without the best players are like vegetables without "tasteless" salt. Big clubs have a lot of finances so it's natural to get the best players and as much as possible they are always used in big competitions, not just in domestic competitions, so it's really strange when we see big clubs playing. without the best players, this is very detrimental for the club itself unless a key player is injured.