Can Sanchez play in the Champions League if we sign him in January? Or will Arsenal playing in the Europa League fuck him over?
He'd be cup tied but there is an exception. From Wikipedia:
"
UEFA operates European club football competitions, primarily the
UEFA Champions League and
UEFA Europa League.
[note 1] UEFA's regulations state that, with a few exceptions, players who play in any European club competition are subsequently cup-tied with respect to all European football for the remainder of the season.
[1]
The main current exception is the
UEFA Super Cup, contested by the winners of the previous seasons' Champions League and Europa League. As this is effectively a prestige
friendly with only one round, the cup-tied rule is seen as unnecessary. Representing a club in this fixture does not affect a player's eligibility in other UEFA competitions.[
citation needed]Similarly, cup-tying did not apply to matches in the now-defunct pre-season
UEFA Intertoto Cup, up to the semi-final round.
A further exemption applies: one player per club who would normally be cup-tied can be registered and eligible to play, so long as his previous club did not field him in the same competition. This means that players can represent two different clubs in the Europa League and Champions League, but only a maximum of one player per club. However, if the first club switches into the same competition (e.g. transfers from the Champions League to the Europa League through finishing third in the group stage), the transferred player becomes ineligible."