Current Zurich Premiership regulations stipulate that a maximum of two non-European Union players can appear for a team at any one time, but the landmark court ruling now means that clubs across the European Union can field as many foreign players as they wish, after handball goalkeeper Maros Kolpak successfully challenged Germany's similar policy.
The rule applies to the more than 100 or so countries who have formal agreements with the EU, including the likes of South Africa and the rugby-playing nations in the Pacific Islands.
Former England wing Hopley, who is now Chief Executive of the Professional Rugby Players' Association, told Planet Rugby: "Unfortunately we have to play by the rules."
The new ruling will not just affect the English league, with Heineken Cup, Celtic League, French championship and Italian league matches among others to come under the same jurisdiction.
"I'm very concerned for young English players and their jobs," said Hopley.
"While the England team is doing well at the moment, having a big influx of foreign players into the league in the long-term will likely have an effect on the national squad."
Foreigners coming into European Union countries to play rugby will still have to meet the criteria needed to obtain a work permit, but once that hurdle is overcome then players from major rugby exporters such as South Africa could well be the beneficiaries in their numbers, with their weak currency potentially lowering their asking price.
"It's happy times for South Africans wanting to come to Europe at the moment to play rugby," said Hopley. "But unfortunately we as a sport will just have to work around it."
He also ruled out any suggestion that the Zurich Premiership clubs could adopt a gentlemen's agreement not to field more than a certain amount of non-EU players in their match-day squad of 22.
"We tried that last year when the Rugby Football Union (RFU) said that a certain amount of English players had to play in each team. One of our members challenged it and the RFU were forced to drop it," said Hopley.
The former Sevens World Cup winner also said that the can of worms opened by the litigation of the Slovakian handball goalkeeper could well have wider implications.
"This could open up all sorts of issues. It has thrown out the rationing of foreign players, and it is eminently possible now that someone could legally challenge the Zurich Premiership's annual salary cap. It could cause a rethink," he said.
By Mark Smith