Gillespie said Leyland and incoming freshman John Alexander, drafted in the eighth round by Tampa Bay out of Glendora High, are the two players he will have to worry about signing before the Aug. 15 deadline.
Summers, an All-American and the Big West Conference Pitcher of the Year, signed with the Minnesota Twins. Gillespie said his minor-league assignment had yet to be announced.
Hillman, who also starred at Orange Coast College, was an 18th-round pick of the Philadelphia Phillies. The UCI outfielder is playing for the Phillies' rookie league team in Clearwater, Fla.
Hernandez, a two-time first-team All-Big West performer who was the conference's Defensive Player of the Year as a third baseman in 2011, is playing for the Angels' rookie league squad in Arizona. He was chosen in the 27th round.
Taylor Sparks, a UCI recruit out of St. John Bosco High who was chosen in the 24th round by the Cleveland Indians, is expected to play at UCI, Gillespie said.
Joining Leyland in the Cape Cod League are catcher Ronnie Shaeffer and pitcher Andrew Thurman.
Gillespie said Thurman, who will be a sophomore next season, pitched against Shaeffer, who will be a senior catcher who earned first-team all-conference honors as a junior, last week.
Shaeffer, the MVP of the 2011 Los Angeles Regional won by the Anteaters at UCLA's Jackie Robinson Stadium, was one for 16 through Thursday, typical of what Gillespie said was a rough start offensively for all Anteaters.
"I think the position players we have out there are hitting less than .100 combined," Gillespie said. "It's the most amazing thing. Nobody can get a hit."
Leyland was three for 15 and second baseman Tommy Reyes, preparing for his senior season by playing with Brainerd Lakes (Minn.) of the Northwoods League, was five for 30 with four RBIs through Thursday.