After winning eight out of nine and three consecutive series to move into first place in the American League East, the Rays have lost two straight and fell into a tie for second with the Orioles at 16-14, two games behind the first-place Red Sox.
Shields assumed all the blame afterward.
"I felt after [Friday night's loss] I needed to pitch better tonight," Shields said. "And I disappointed the bullpen, I disappointed the team. I'm not too happy."
Shields got off to a rough start as the Red Sox's first four hitters reached base on hits. Included in those safeties were an RBI double by David Ortiz and a two-run single by Manny Ramirez, who recorded his first RBIs since April 19 to give the Red Sox a 3-0 lead.
"It was pretty much terrible," Shields said. "It was one of those games where that first inning, I felt like I made some pretty good pitches. Ellsbury, jam-shot cutter [single into center field]. Two-seamer to [Dustin] Pedroia [single to left], got behind in the count to Big Papi [double to left]. And things started rolling in the first inning. After that, I don't think I pitched very well.
"I was getting the ground balls I needed, but they were finding holes. That's a good hitting squad over there. I felt I made pretty good pitches today, but it's not my day today."
Rays manager Joe Maddon said he did not think Shields looked comfortable Saturday night.
"From the side, you look at the velocity readings, they were normal, but overall command," Maddon said. "It's a tough night to pitch, too, because it's kind of cold. But I just thought from the side, he just looked not comfortable. Not being totally out of whack, but just not being able to make the pitches he normally makes."
Gabe Gross hit a two-run homer in the second off Josh Beckett to cut the lead to 3-2, but Beckett settled in to limit the Rays to four runs over eight innings and pick up his third win of the season, while the Red Sox continued to pound Shields.
Dustin Pedroia had an RBI single in the second and Julio Lugo doubled home another in the third to push the lead to 5-2. Evan Longoria answered with an RBI single in the fourth, and the Rays had an opportunity for a big inning when they loaded the bases with one out. But when Nathan Haynes hit what appeared to be a sacrifice fly to left field, Ramirez threw out Carlos Pena at the plate to end the threat.
Shields surrendered back-to-back doubles to Pedroia and Ortiz to start the fourth. He recovered to retire the next two hitters before walking Kevin Youkilis on his 98th pitch and getting the early hook.
"I think today was just their day," Shields said. "I felt like I made pretty good pitches. Once I got  pitches in three innings, or whatever, I got a little tired at the end. Walked Youkilis and got taken out. It's just one of those days where it's their day and you've got to tip your cap."
Shields fell to 3-2 for the season, while his ERA swelled from 2.54 to 3.80 after experiencing his shortest outing since July 22, when he lasted just 3 1/3 innings against the Yankees in New York.
"It was frustrating," Shields said. "After last night, I felt like I needed to come in here and do a good job. I didn't do that at all. I hurt our bullpen. I hurt our team today. I'm going to go on and try to do something special the next game."