Farrell noted that the decision was made at the beginning of spring training and isn't a result of the strained left elbow sustained by David Price, who was the team's 2016 Opening Day starter.
"We had three candidates that were certainly worthy and capable," Farrell said, referring to both Price and recently acquired left-hander Chris Sale.
"But I think there's a lot to be said for the year Rick has had previous, the leader that he's become on our team and the dependable pitcher that he is."
Porcello, who will be opposed by Pittsburgh ace Gerrit Cole on April 3 at Fenway Park, posted a 22-4 mark with a 3.15 ERA last season en route to being named the American League Cy Young Award winner. Porcello, 28, led the league with a 5.91 strikeout-to-walk ratio and came in second in walks plus hits per innings pitched (WHIP) at 1.01.
Sale is expected to start Boston's second game on April 5. Price is likely to start the season on the disabled list.