"We tried to move Dunn here a little bit earlier but his wife had a baby," said manager Ozzie Guillen. "That's the reason we couldn't move him earlier than this."
Dunn's wife gave birth to a son on Saturday in Miami.
He appeared in 12 games with the Marlins earlier this season before being optioned to Triple-A on May 24. With Miami, Dunn had a 9.58 ERA in 12 games with 17 strikeouts in 10 1/3 innings.
In 12 appearances with New Orleans, he was 1-1 with a 4.58 ERA and 24 strikeouts in 17 2/3 innings.
"I was pounding the strike zone and making them swing the bat to make them put it in play instead of before, where I was trying to guide a pitch," Dunn said. "I want to force contact instead of pitch to contact. The whole time I was down there I only had one walk. I was just going right at people, able to finish them off early."
Dunn arrived at Fenway Park around 2:30 p.m. ET on Thursday.
Hatcher was hit hard in a relief appearance in Wednesday's 15-5 loss. The right-hander gave up a grand slam to David Ortiz and a homer to Jarrod Saltalamacchia. He allowed four runs in 1 2/3 innings.