The club have confirmed that Diaz returned a positive Covid-19 test and is now in quarantine at home after missing training on Tuesday.
Spanish newspaper Marca are reporting that Diaz returned a positive test on his return from holiday following a short trip after the conclusion of the LaLiga season.
Real Madrid striker Mariano Diaz has tested positive for coronavirus after returning to the club
Diaz returned from a short holiday after the LaLiga season finished and was tested on Monday
The Spanish giants are due to play Manchester City in the Champions League next Friday
A club statement read: ‘After the COVID-19 tests carried out individually on our first football team yesterday (Monday) by the Real Madrid Medical Services, our player Mariano has given a positive result.
‘The player is in perfect health and complying with the sanitary isolation protocol at home.’
The Spanish giants are due to play the second leg of their last-16 Champions League tie at the Etihad Stadium on August 7.
Mariano will now have to undertake a two-week quarantine period, ruling him out of the game against the Premier League side.
The news comes amid the ongoing row between the UK and Spanish Governments after the UK re-imposed the requirement for travellers arriving from Spain to spend 14 days in quarantine.
But the positive test is now likely to bring the game into question, particularly if more Real players test positive in their next round of testing.
It will also ramp up the pressure on the Government, with the Spanish champions currently allowed to the travel and play despite the UK’s new quarantine rules due to exemptions surrounding their ‘bubbled environment’.
It remains to be seen how the positive test will impact the game against Manchester City
The UK re-imposed requirement for travellers arriving from Spain to spend 14 days in isolation
It was decided on Saturday that the UK should remove Spain from its list of safe countries to travel to after fears were raised that the European country was experiencing a second wave of coronavirus infections.
Ministers decided to reimpose quarantine travel restrictions on Spain from midnight on Sunday after it emerged 10 Britons had returned home from the country with coronavirus.
The UK’s chief medical office, Professor Chris Whitty, led a meeting with six senior ministers to discuss the worsening situation in Spain and insisted action had to be taken.
It comes just over a week after Real wrapped up their LaLiga campaign, which followed a strict set of protocols to ensure the season could be concluded after the coronavirus pandemic stopped play.
Real won the LaLiga title, finishing five points clear of rivals Barcelona, but face an uphill task to eliminate City from the Champions League.
Pep Guardiola’s side hold a 2-1 aggregate lead from the first leg at the Bernabeu way back in February. Captain Sergio Ramos was also sent off so he will be missing for the return leg.
Mariano is far from a key player for Zidane. He has played just seven times for Real this season across all competitions and has scored one goal.