Here bro, from Uefa rules. List A only consisted of Peña and Mats, Mats got injured and was then replaced.
"If a club cannot count on the services of at least two goalkeepers registered on List A because of long-term injury or illness, the club concerned may temporarily replace the goalkeeper concerned and register a new goalkeeper at any time during the season and complete the official registration of List A with a goalkeeper fit to be fielded. An injury or illness is considered long-term if it lasts at least 30 days as of the day the injury or illness occurred. If the goalkeeper recovers before the end of this 30-day period, the player must nonetheless remain off List A until the end of such period. The new goalkeeper does not have to be locally trained, even if the replaced goalkeeper was. The club must provide UEFA with the necessary medical evidence in one of UEFA's official languages. UEFA may require further medical examination of the goalkeeper by an expert appointed by UEFA at the club's expense. Once the injured or ill goalkeeper is fit to be fielded again, he can resume his position in place of his substitute. The return of the original goalkeeper must be announced to the UEFA administration 24 hours before the next match in which the original goalkeeper is due to play."
Read your own post again and ovelay it on Articles 30.01 and 31.03
In summary, it is stating that you can only register a new goalkeeper when one is injured or ill while not deregistering the one that is injured. You can inform them that the one you replaced with another keeper is now back and ready to play on 24 hours.
You can NOT register a new player in 24 hours if he was never registered for the 2nd phase of the competition as per Article 32. Additionally, Article 31.13 allows you to register 5 goalkeepers and Mats was not among the 5 registered by Barca.
He CAN not suddenly be available to play UCL again unless one of those 5 is injured. Simple. If he was still part of the 5 registered by Barca but club had to bring in a 6th because he was injured or ill, he could still play with a 24 hour's notice.
I am sure the Team managers know this, that is why no one is giving that discussion any attention.