Dale Coyne has admitted that his IndyCar team will need share its second seat between two drivers as no one has come forward with a full budget for the season.
Following Ed Jones' surprise departure from the team to join Chip Ganassi Racing, Coyne has been searching for someone "with talent and money" to partner Sebastien Bourdais.
Zachary Claman De Melo, race winner in Indy Lights in 2017, who made his IndyCar debut with Rahal Letterman Lanigan Racing in the Sonoma finale was the favourite for the second seat.
Following a successful test as Sebring last week, Formula V8 3.5 champion Pietro Fittipaldi is now also in contention.
Coyne has admitted that the likelihood of a candidate emerging with a full-season budget is unlikely, and so the team will probably need to split the second car.
"I think we'll have to run two - at least two - in that car. I don't see anyone coming up with a full budget, and I need to have deals done by the end of next week, because then we test at Sonoma," said Coyne.
"I don't know who's testing there. It could be Zachary, could be Pietro, could be a third guy. I don't know yet."
Coyne would not disclose who the third contender was, but said: "The chance of a [Jack] Hawksworth deal... that's gone away a little bit. No, this other driver would be someone who hasn't been in IndyCar or Indy Lights, but does have open-wheel experience."
Following two days of testing at Sebring - Claman de Melo on Wednesday, Fittipaldi on Thursday - Coyne said he had been impressed with both drivers.
"Zachary knew what he was doing in the car, he adapted well, understood the changes we were making, and we were very pleased.
"As far as lap times were concerned, it was very tight between the two of them - within two tenths - so I was happy with Pietro, too. He adapted well. I think the Formula 3.5 is a good car to prepare for IndyCar - about the same size and about the same downforce now that we've lost a bunch.
"He had to understand how the tyre performance peaked - that's the tricky bit with these cars, and it takes experience. But he took the whole thing very, very seriously - got there early, left late, studied data, watched onboard videos. And his feedback was very, very good.
Coyne admitted De Melo's oval experience could be a factor in splitting up which races the drivers contest, but said it was no guarantee and described it as "a work in progress".