UEFA have made a change to their squad registration rules that could allow Arsenal to call up Gabriel Jesus or others for the Champions League in certain specific circumstances.

Arsenal submitted their List A squad for the Champions League recently, notably including 15-year-old Max Dowman on their squad list, but leaving out senior professional Gabriel Jesus.
In the past, the senior squad list has been fixed for the entire Group Stage or League Phase, with changes only possible ahead of the knockout stages. That meant there was no way for someone like Jesus to be added back into the squad until February at the earliest.
But a late alteration to the rules now creates a possibility that Jesus could still represent Arsenal before the League Phase ends.

UEFA have confirmed that they’ve approved an amendment to the rules to allow the temporary replacement of a maximum of one outfield player with long-term injury or illness during the League Phase until Matchday 6 of the competition.
So if Arsenal suffered an injury to Viktor Gyokeres, for example, Jesus would then become eligible to be added back into the squad and called up for a game.
Explaining the change, UEFA wrote that they wanted to ensure “that squad lists are not unfairly reduced and players are safeguarded from additional workload pressure”.

Of course, Jesus isn’t currently fit to play anyway, as he continues to recover from an ACL injury. That will be a large part of why Arsenal opted not to include him in the squad.
But there is now at least a possibility that he could represent the Gunners in the League Phase if his injury rehabilitation goes well.
The rule change also doesn’t just apply to Jesus. Arsenal have had to leave out some youth players due to a lack of available spots and restrictions on including u21 players who have been at the club for less than two years.
The likes of Demiane Agustien, who was involved in first-team training on Monday, can’t currently play in the Champions League.
Agustien isn’t in Arsenal’s senior squad list, and he can’t be named on the u21 list having just arrived this summer. But he could be added to the senior list if Arsenal lost a player to injury or illness.