Rodrygo is a good player and he is one of the most gifted player in Real Madrid Squad, he is not a player that any coach would bench but due to the coach plan he decided to use Mbape and Vinicius, if Rodrygo can be patient this season and perform well he will regain his position in the team, Vinicius performance has not been good enough to help Real Madrid since last season, I don’t know maybe it is because of the new contract agreement that hasn’t been renew is affecting Vinicius, his performance is very poor even in preseason games, and from look at the contract issues everything looks like Real Madrid will soon sell Vinicius.
Xabi Alonso trusts Vinicius because of his skills and speed, but he stated that he is counting on every player, and he is counting on Rodrygo 100%. Xabi Alonso will start to bench Vinicius if he does not play the way well. A true Real Madrid fan would know that since the arrival of Mbape, Mbape and Vinicius' performance has not been too good, but whenever Rodrygo and Mbape play together, they have more connections than Mbape and Vinicius, so Rodrygo's decision to stay in the team is the best.
The manager's comment makes it clear that Rodrygo will be in the starting XI. Now, who will sit on the bench if Rodrygo is in the starting XI? The manager may have already planned this, Vinicius Junior is one of Real Madrid's best players, so it makes no sense to put him on the bench. However, as you said, if Vinicius Junior performs poorly, he may have to sit on the bench, this is not only the case of Vinicius Junior but also of all the footballers in Real Madrid.
A team will not win because of the name, but because of the performance of the player on the field. If Mbappe cannot perform well, then Mbappe will be kept on the bench, if Vinicius Junior performs poorly, then he will be kept on the bench, and if the goalkeeper does poorly, then the manager will decide to keep him on the bench.
However, all the rumors in the transfer market have finally been proven wrong and Rodrygo is staying at Real Madrid and is getting a place in the starting XI.