Where do posters get the idea that U21's are unlimited from?
They're only unlimited if they're on the B list (which means that they've been with us for 2 years) - otherwise they take up a full squad place.
http://www.uefa.org/MultimediaFiles.../Regulations/02/35/87/89/2358789_DOWNLOAD.pdf
Article 42 Player eligibility
42.01 In order to be eligible to participate in the UEFA club competitions, players must be registered with UEFA within the requested deadlines to play for a club and fulfil all the conditions set out in the following provisions. Only players that are duly registered with UEFA by means of list A or B can validly serve pending suspensions.
42.02 Players must be duly registered with the association concerned in accordance with the association’s own rules and those of FIFA, notably the FIFA Regulations on the Status and Transfer of Players.
42.03 Each player taking part in the competition must be in possession of a player’s registration licence issued by his association or a valid passport or identity card, each containing his photograph and giving full particulars of his date of birth (day, month, year). The referee or a UEFA match officer may ask to see the passports/identity cards of the players listed on the match sheet.
42.04 All players must undergo a medical examination to the extent provided for by the UEFA Medical Regulations.
42.05 The club bears the legal consequences for fielding a player who is not named on list A or B, or who is otherwise not eligible to play.
42.06 The UEFA administration decides on questions of player eligibility. Challenged decisions are dealt with by the UEFA Control, Ethics and Disciplinary Body.
42.07 As a rule, a player may not play in a UEFA club competition (i.e. UEFA Champions League and UEFA Europa League, but not UEFA Super Cup) for more than one club in the course of the same season. Exceptionally, however, a player who has been fielded in the first, second, third qualifying round or the play-offs of the UEFA Champions League or UEFA Europa League is entitled to play in the UEFA Champions League or UEFA Europa League for another club as from the group stage. Moreover, as from the start of the round of 16, a player can be registered in accordance with Paragraph 44.01 to Paragraph 44.04. A substitute player who has not been fielded is entitled to play for another club competing in the UEFA Champions League or UEFA Europa League in the course of the same season, provided that he is registered with the UEFA administration in accordance with the present regulations.
Article 43 Player lists
43.01 Each club is responsible for submitting an A list of players (List A) and a B list of players (List B), duly signed, to its association for verification, validation, signature and forwarding to UEFA. These lists must include the name, date of birth, shirt number and name, nationality and national registration date of all players to be fielded in the UEFA club competition in question, as well as the surname and first VII – Player Registration 39 name of the head coach. In addition, the lists must include the confirmation by the club’s doctor that all players have undergone the requested medical examination; the club’s doctor is solely responsible for ensuring that the requested players’ medical examination has been duly performed.
43.02 No club may have more than 25 players on List A during the season, two of whom must be goalkeepers. As a minimum, eight places are reserved exclusively for "locally trained players" and no club may have more than four "association trained players" listed on these eight places on List A. List A must specify the players who qualify as being "locally trained", as well as whether they are "club trained" or "association-trained". The possible combinations that enable clubs to comply with the List A requirements are set out in Annex G.
43.03 A "locally trained player" is either a "club-trained player" or an "association trained player".
43.04 A "club-trained player" is a player who, between the age of 15 (or the start of the season during which he turns 15) and 21 (or the end of the season during which he turns 21), and irrespective of his nationality and age, has been registered with his current club for a period, continuous or not, of three entire seasons (i.e. a period starting with the first official match of the relevant national championship and ending with the last official match of that relevant national championship) or of 36 months.
43.05 An "association-trained player" is a player who, between the age of 15 (or the start of the season during which the player turns 15) and 21 (or the end of the season during which the player turns 21), and irrespective of his nationality and age, has been registered with a club or with other clubs affiliated to the same association as that of his current club for a period, continuous or not, of three entire seasons or of 36 months. 43.06 If a club has fewer than eight locally trained players in its squad, then the maximum number of players on List A is reduced accordingly.
43.07 List A has to be submitted by the following fixed deadlines: a. 20 June 2016 (24.00CET) for all matches in the first qualifying round; b. 7 July 2016 (24.00CET) for all matches in the second qualifying round; c. 21 July 2016 (24.00CET) for all matches in the third qualifying round; d. 8 August 2016 (24.00CET) for all matches in the play-offs; e. 1 September 2016 (24.00CET) for all further matches from the first match in the group stage up to and including the final.
43.08 For the three qualifying rounds and the play-offs, a club may register a maximum of one new eligible player on List A after the above-mentioned deadlines, provided the quota of locally trained players is respected. Such registration must be completed by 24.00CET on the day before the relevant first-leg match and the club’s association must confirm in writing that the new player is eligible to play at domestic level at this time. 40 VII – Player Registration
43.09 If the registration of such a new player causes the number of players on List A to exceed 25, the club must remove a currently registered player to reduce the squad to 25 players again.
43.10 Each club is entitled to register an unlimited number of players on List B during the season. The list must be submitted by no later than 24.00CET on the day before the match in question.
43.11 A player may be registered on List B if he is born on or after 1 January 1995 and has been eligible to play for the club concerned for any uninterrupted period of two years since his 15th birthday by the time he is registered with UEFA. Players aged 16 may be registered on List B if they have been registered with the participating club for the previous two years without interruption.
Bottom Line:
Leroy Sane and Oleksandr Zinchenko take up full squad places if picked for the Champions League.