Yes he would have to be deemed to be medically unable to play this season. If it was confirmed now however, we could sign a player before the end of the SSP period on Feb 21.
Here's the laws of the game part covering it:
26. Inactive List
26.1 Inactive List (a) A Club may apply to the General Counsel or Executive General Manager of Football to transfer an AFL Player suffering a season ending injury or has retired from its AFL Primary List or Category A Rookie List to its AFL Inactive List. (b) A season ending injury means an injury or illness suffered by a Player which, in the opinion of the AFL Medical Director, after consultation with the Club Medical Officer, will prevent or is likely to prevent a Player, having due regard to their health and safety, from participating in Matches for the remainder of the relevant AFL Season.
26.2 Approval of the AFL (a) A Player may only be transferred to the Inactive List with the Player’s consent and with the approval of the General Counsel. (b) Where a Club proposes to transfer a Player to the Inactive List, the following procedure shall apply: (i) an Officer of the Club shall as soon as practicable, complete an “Application for Inclusion on Inactive List” in the form prescribed in Schedule 1 as Form 43 (“the Application”); (ii) the Officer of the Club and the Club Medical Officer shall sign the Application, where applicable; (iii) the Player shall sign the Application as evidence of their consent to being transferred to the Inactive List; (iv) the Officer of the Club shall lodge the Application with the General Counsel, together with supporting evidence which verifies that the Player is suffering from a season ending injury or has retired; (v) the General Counsel may in their absolute discretion, approve or disapprove the transfer of the injured Player to the Club’s Inactive List or require such further medical evidence, including without limitation a medical report from a duly qualified medical practitioner approved by the General Counsel, prior to determining whether to approve or disapprove the transfer; and (vi) where the General Counsel gives their approval, the injured Player shall from that time be deemed to be included on the Club’s Inactive List. (c) The transfer of a Player to a Club’s Inactive List under this Rule 26.2 shall not take effect until the General Counsel has notified their approval under Rule 26.2(a). Where the General Counsel does not approve the transfer of the injured Player to the Club’s Inactive List, they shall notify the Officer of the Club and the Application shall be of no effect.
26.3 Eligibility Subject to Rule 26.5, a Player who is transferred to the Inactive List is ineligible to play in the AFL Competition or in any other Australian Football competition for the period 100 from the date that Player is transferred to the Inactive List until the conclusion of the relevant AFL Season.
26.4 Replacement Player (a) Where a Player is transferred to the Inactive List a Club may replace that Player by selecting a replacement player during the Pre-Season Supplemental Selection Period or at the mid-AFL Season Draft Selection Meeting. (b) A Club is unable to replace an AFL Player transferred to its AFL Inactive List after the conclusion of the mid-AFL Season Draft Selection Meeting.
26.5 Transfer from Inactive List to Primary List (a) A Club may, subject to that Club having an available List position, make written application in the form prescribed in Schedule 1 as Form 43A to the General Counsel for the transfer of a Player from its Inactive List to the Club’s Primary List or Category A Rookie List in the relevant AFL Season that Player was transferred to the Inactive List provided that the Club Medical Officer or the medical practitioner who has treated the Player certifies that the Player has recovered from their injury sufficiently to allow the Player, having due regard to their health and safety, to play in a Match and such certification is lodged with the application. (b) The General Counsel may in their absolute discretion, approve or disapprove an application under Rule
26.5(a) for the transfer of a Player to the Club’s Primary List or require such further medical evidence, including without limitation, a medical report from a duly qualified medical practitioner approved by the General Counsel, prior to determining whether to approve or disapprove the transfer. (c) The transfer of a Player back to a Club’s Primary List or Category A Rookie List under Rule
26.5(b) shall not take effect until the General Counsel has notified their approval. Where the General Counsel does not approve the transfer of the Player back to the Club’s Primary List or Category A Rookie List, they shall notify the Officer of the Club and the Application shall be of no effect.
26.6 Consultation The General Counsel may consult with the AFL Medical Director prior to exercising their discretion to approve or disapprove a transfer under Rule 26.2 or 26.5