With top prospect Matt Chapman set to be officially called up on Thursday, the A’s are preparing to designate third baseman Trevor Plouffe for assignment, The Chronicle has learned.

Plouffe, signed to a one-year, $5.25 million deal during the offseason, is batting .214 with seven homers and 14 RBIs and six errors in 58 games and his playing time had been diminished the past two weeks. He started just one game on the team’s just-completed road trip to Tampa Bay and Miami.

Chapman, 24, was the A’s first-round pick in the 2014 draft, 25th overall. He is likely to be the everyday third baseman; Ryon Healy, who has been playing the position more often lately, has made eight errors in 18 starts there. Healy probably will return to splitting time between the designated hitter spot and first base.

Susan Slusser is a San Francisco Chronicle staff writer. Email: sslusser@sfchronicle.com Twitter: @susanslusser