Jeff Samardzija was already having an August to forget.
Boston Red Sox right fielder Rusney Castillo only made it worse Monday night.
Castillo homered and drove in a career-high five runs, Joe Kelly won his fifth straight outing and the Red Sox beat the Chicago White Sox 5-4 on Monday night.
Castillo hit a three-run homer in the second inning and went 3 for 4, finishing a triple shy of the cycle. His sixth-inning double to the right-center field gap drove in two runs and extended Boston's lead to 5-1.
Samardzija (8-10) dropped a career-worst fifth straight decision. After giving up Castillo's two-run double, he allowed the next two hitters to reach before being pulled after surrendering his second walk.
Samardzija struck out seven and allowed five earned runs and eight hits. Castillo did most of the damage.
"He got me today," said Samardzija, who has allowed 28 runs in 28 innings in August. "It's unfortunate.
"I felt good. I thought I had good stuff today. I was around the zone. Just a homer and a double."
That was plenty for Kelly.
Kelly (7-6) allowed five hits and two runs over 7 1-3 innings while walking one and striking out three. He became the first Red Sox pitcher since Clay Buchholz in 2013 to win five straight starts.
Jean Machi pitched the ninth for his second save in two opportunities.
The White Sox, who had won three of their last four, cut the deficit to 5-2 in the sixth on Melky Cabrera's RBI groundout.
Chicago tacked on two runs in the ninth, but couldn't rebound from falling behind early. Samardzija took the crux of the blame.
"It's been frustrating," Samardzija said. "I can just speak for myself, the last few times out when I've given up runs we've scored runs and when you don't, we don't. That sums it up."
The Red Sox took a 3-1 lead in the second inning when Castillo drove Samardzija's slider over the center-field fence for a three-run homer. The two-out blast came two batters after Samardzija drilled Hanley Ramirez in the back after quickly retiring the first two batters.
Brock Holt followed with a single before Castillo's homer, which extended his hitting streak to nine games.
"This guy has been dynamite," Red Sox interim manager Torey Lovullo said. "He has responded to a lot of the challenges. I know that he's been coming on as of late and letting his game speak for itself. There is an ease and a rhythm to him."
Chicago jumped out to a 1-0 lead on Jose Abreu's solo homer, his 24th, but it wasn't enough to end Samardzija's slide.
"I thought he had a good game except the one guy (Castillo) got him twice," White Sox manager Robin Ventura said.