Article 43 in the competition's lengthy regulations guide states each club must name their 25-man playing list - made up of a set number of homegrown players - before the group stages.

But Article 44 states changes can be made if newly registered players meet certain requirements before the knockout stages.

Crucially, only players who haven't played in the Champions League for another club in the same season can be registered.

Sanchez has turned out in the Europa League for Arsenal, netting the only goal of the game in a 1-0 win over Koln back in September.

But the fact the Gunners failed to qualify for the Champions League mean he can now run out in the competition this season for Manchester United.

That's providing he is now registered before the February 1 deadline of midnight. United face Sevilla in a two-legged last-16 knockout clash next month.

Article 44.01 reads: "For all matches from the start of the round of 16, a club may register a maximum of  three  new  eligible  players  for  the  remaining  matches  in  the  current competition. Such registration must be completed by 1 February 2018 (24.00CET) at the latest. This deadline cannot be extended."

Article 44.02 then adds: "One player from the above quota of three who has been fielded in a UEFA club competition  group  stage  match  for  another  club  in  the  current  season  may exceptionally be registered, provided that the player has not been fielded:

a. in the same competition for another club; or

b. for another club that is currently in the same competition."

Express Sport understands Sanchez is set to complete a £350,000-a-week switch to Manchester United in the next 48 hours.

The Chile striker was left out of the squad for their shocking 2-1 defeat at Bournemouth and Gunners manager Arsene Wenger admitted he expected the deal to be completed imminently.

United have edged ahead of rivals Manchester City in the bidding for Sanchez, being prepared to match Arsenal’s £35million valuation this month, with Henrikh Mkhitaryan possibly going the other way as part of the deal.

City had told Sanchez that if he came in the summer, as a free agent when his contract runs out, his potential wages at the Etihad could be doubled – but United are prepared to move now.

Wenger, who insisted that he would be given the cash to replace Sanchez in this transfer window, said: “Alexis did not refuse to play. He would have played but it’s a difficult period for him.

“He’s always been committed until now but he could have moved yesterday, today or not. It will be decided in the next 48 hours.

“It was difficult for him because he was on stand-by. That’s why I decided not play him.

“If Sanchez goes we need to replace him. If players like him go, we have to respond.

"At the moment, it’s the transfer period and in this kind of period, you try to use players who are completely focused on being at the club. It could happen today, tomorrow or not at all.

“I have been assured that Alexis will be replaced but there’s nothing concrete there yet.”