It wasn't, of course.
But English teams' squads were made up of British-born players... and Brit born players couldn't represent Man Utd unless they were English or in the three allowed. So Denis Irwin, Roy Keane, Brian McClair, Mark Hughes, Ryan Giggs, who were home grown but still considered foreigners were added to names like Peter Schmiechel, Eric Cantona, Andrei Kancheslskis. of whom Sir Alex could only name any 3.
It's a fact that English clubs suffered the most from this foreigner rule.
English clubs couldn't stop winning the European Cup from the late 70s and into the 80s... but then it all stopped in the 90s with the rebrand to the Champions League and the introduction of the foreigner rule. When the foreigner rule stopped in 1997; Man Utd won the CL in 1999, Liverpool in 2005, United in 2008, Chelsea in 2012. In fact from 2004 to 2012 there was an English team in EVERY final. The only years that are missing from the success of English teams in the European Cup are the 90s when the foreigner rule was in place...because the rule didn't include British players (who were classed as home grown in our domestic settings) as home grown....