In August, Castillo, 27, signed a deal reportedly worth $72 million over seven years with the Red Sox.
In 40 plate appearances over 10 games this season, Castillo posted a .333 batting average with two home runs and six RBI.
Boston manager John Farrell commented on Castillo's progress on Tuesday. From masslive.com:
"There's an aggressive swing there," Farrell said. "I think the more we get around him, the more we see him in games, we're getting more familiar with him. He feels more relaxed, more confidence because he has the number of at-bats he was able to compile last year. I think a different guy, a more comfortable guy, than who was brought to us last September."
The Red Sox went 71-91 last season and finished in last place in the American League East.
Rusney Castillo has a strained left oblique. He will miss at least a week. Betts, Bradley will get primary reps in center field.— Brian MacPherson (@brianmacp) March 4, 2015