Jonny May's move from Leicester to Gloucester highlights the "confusion and ambiguity" of the current situation regarding player contracts, according to a top sports lawyer.
Gloucester have signed May from 1 July and expect him to be available to play immediately.
However, the current season is likely to be extended well past that date.
Lawyer Richard Cramer says preventing May from playing would be a "restraint of trade".
No games have been played in the Premiership since 8 March after the season was suspended because of the coronavirus outbreak. Leicester and Gloucester are still due to meet this season, in round 17.
Cramer told BBC Sport: "This has caught Premiership Rugby out a little bit. The big debate is what Premiership Rugby do now do in terms of registration.
"They have got a bit of a headache now because I don't think the rules provide for this scenario - and to stop a player playing and plying his trade would be a restraint of trade."
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Numerous players have lined up similar 'end-of-season' moves to May, such as fellow England star Kyle Sinckler, who has signed for Bristol from Harlequins.
Centre Sam Hill is set to move from table-toppers Exeter to second-placed Sale.
All standard contracts start on 1 July and end on 30 June, and Cramer believes the players would have a right to fulfil the original terms of their contracts and play for their new club from July, regardless of the unprecedented impact of the coronavirus pandemic.
"We have a little bit of ambiguity in terms of the process of players coming out of contract and moving into new contract, but the starting point must be that you can't stop a player playing," he added.
"It's a bit embarrassing for the sport when we are all now trying to fathom how it is going to pan out.
"What should have happened is the RFU and Premiership should have set some guidelines, because it is going to leave a lot of questions hanging over the sport generally."
Cramer added that a club signing a player may also have recourse to prevent him staying with his current club beyond 1 July.
"You've got a scenario where a player is coming to the end of their contract on 30 June and they may feel they want to do the 'honourable' thing and stay with their club [until the end of the season], but the new club may want him on board immediately," he said.
"They may not want their incoming prized possession playing [for the old club] in July when technically he is their player. It is definitely going to cause some confusion.
"What I would imagine now is off the back of this the RFU, Premiership and RPA [players' union] are going to have to get their heads together pretty quickly to create a new set of rules and guidelines."
The situation also throws up questions over the salary cap, and where the wages of new players fit into the regulations.
"From Gloucester's point of view, is Jonny May's salary - that they are going to pay from 1 July - a salary in the 2020-21 season, or a salary in the 2019-20 season?" Cramer asked.
"These are issues for the salary cap regulator, and is another headache which was totally unforeseen."
Premiership Rugby is exploring the possibility of rescheduling the Twickenham final deep into the summer after further postponing the season.
The campaign was initially called off until 24 April because of coronavirus. There are nine rounds of the regular season remaining, with the Twickenham final originally planned for 20 June.
But despite the further delay, Premiership Rugby says it remains committed to "completing all games" in the 2019-20 season.