"He went out to the bullpen and then it was like five minutes before we were ready to start that I got the news he wasn't going to answer the bell," manager Ron Johnson said. "Keith came by and said, 'I'm sorry, man, but I can't go out there right now.' So, that was it. All I heard was that his back was stiff."
That was confirmed by PawSox pitching coach Mike Griffin, who observed Foulke along with Boston's assistant trainer Mike Reinold.
"He had some stiffness so he just didn't want to try it," Griffin said. "He warmed up. He stretched out and did his long toss. Then, he got up on the mound and he felt tight.
"He was here to pitch his inning and wasn't able to do so."
Foulke also missed a week in June because of a sore back.
Foulke wasn't talkative as he exited McCoy Stadium and climbed into a black sedan for the drive back to Boston.
When asked he if felt something was amiss while warming up, Foulke said "I'm not pitching. You can call it whatever you want. ... We'll let you know. I don't want to talk about it, really."
Foulke pitched one inning Friday night at Class A Lowell against Vermont and retired the side in order on 11 pitches. He had also thrown two simulated innings July 24 in Oakland and a side session prior to a July 26 game against the A's.
He was scheduled to throw 20 to 25 pitches for Pawtucket.
"[Boston] talked about [Foulke] throwing an inning today and two on Tuesday," Johnson said. "We'll see. It's just a back stiffness. I'm sure he'll be back."
Foulke's last outing was June 11 against Texas when he pitched two innings and allowed four runs on seven hits.
Foulke has been limited to 32 innings over 29 relief appearances and is 2-1 with a 5.63 ERA and 23 strikeouts.
Foulke stated earlier this season that his elbow had bothered him last year.
Boston's holds a $7.5 million option on Foulke for next season which includes a $1.5 million buyout clause.
If the Red Sox don't exercise their option, Foulke can vest it himself for $3.75 million.