Yankees general manager Brian Cashman said three additional tests were pending and he assumed they would come back positive.
ESPN reports the three other Yankees position players who are in COVID-19 protocol are Aaron Judge, Kyle Higashioka, and Gio Urshela.
Because Judge was at the All-Star Game with other players, this has triggered contact tracing questions for the Red Sox players.
Loaisiga went on the COVID-19 injured list Saturday, when the Yankees were in Houston, and he did not travel home with the team Sunday.
Cashman said most of the players on the team had been vaccinated, but not all.
While vaccinations do not prevent a person from contracting COVID, it prevents severe effects.
"As of right now, everybody's OK," he said.
Cashman said Major League Baseball had not yet decided whether to postpone Friday's second scheduled game of the four-game series.
"We're in a very fluid situation right now," he said.
This was the eighth COVID-related postponement this season but the first in nearly three months. Also put off were a three-game series that had the New York Mets at Washington from April 1-4, and Atlanta's game at the Nationals on April 5, two Minnesota at Los Angeles Angels games on April 17-18 and a Twins at Oakland game on April 19.
There were 45 regular-season games postponed for virus-related reasons last year but just two were not made up, between St. Louis and Detroit.
New York was among the first major league teams to reach the 85% vaccination threshold to lessen coronavirus protocols such as dropping mask use in dugouts and bullpens.
Despite all those vaccinations, the Yankees had more than a half-dozen positive COVID tests in May, and third base coach Phil Nevin became seriously ill.
New York players were on the field taking early batting practice about 3 1/2 hours before the scheduled start when the Yankees asked media to leave the field while the team conducted COVID testing.
Boston came onto the field as the postponment was announced to take batting practice.
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