Premier League spokesman Dan Johnson said: "We are aware of the situation with Jo and the rules are very clear. At the Premier League AGM held two weeks ago it was agreed that third-party ownership of any player will not be allowed.
"While that situation is common in countries like Spain and Portugal, we agreed that it is not tolerable here.
"We want to ensure no one can accuse us of compromising the integrity of the competition so we have stated that third-party ownership is illegal.
"We believe this is in the best interests of the Premier League and our clubs and we think we are economically strong enough to impose it."