The former Barcelona player has joined up with businessman Paul Richardson in a bid to take over the Blues.
It was initially believed that Laurence Bassini led the chance to buy the club, but his efforts have fallen by the wayside.
Lopez and Richardson now have exclusivity to discuss taking over the Championship side, which they hope will see a deal concluded in the coming weeks.
The pair are now hoping to meet with the recently appointed Eustace to discuss whether he is part of their vision for Birmingham.
The 42-year old was appointed with the expectation that Bassini would be taking over the club.
However, he has only been involved with the Blues since earlier this month so it would be a drastic decision for him to be relieved of his duties.
The former QPR assistant manager replaced Lee Bowyer, who led the side to a 20th place finish in the table last season.
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Birmingham will be aiming to be more competitive over the next 12 months, with the new campaign set to get underway later this month.
Eustace’s side meet Luton Town on the opening day of the new term on July 30.
It would be a remarkable turnaround if Eustace was not chosen as the man to lead the new era at Birmingham City.
Considering he was appointed only a couple of weeks ago, he must surely be given a chance to prove he is the right man for the job.
A public backing from the new owners may be needed to ease the immediate pressure from the coach, as this could be a tricky way to start life as manager of a new team.
Certainly backing in the transfer market will be needed to ensure the club can remain competitive in the Championship this season.