The 33-year-old sinkerballer's Red Sox debut is set for Saturday at 1:10 p.m. ET against the Orioles, and Friday marked his first game with the team.
"It's really nice. This is where I wanted to be the whole time," said Cook, who's pitched all season at Triple-A Pawtucket and had an opt-out clause on Tuesday. "To actually get the call up and be here and be with the guys I was with in Spring Training and be able to make my first start here, it's unbelievable."
Cook went 3-0 with a 1.89 ERA in five starts for Pawtucket. It's not clear how the Sox will use Cook after Saturday -- a six-man rotation remains possible with 20 games in as many days for the Sox starting Friday -- but he said he's open to pitching in relief.
"I wouldn't be opposed. Pitching here and contributing in any way is what I want to do, but you know, my main focus is [Saturday]," Cook said. "I got to go out there, take the ball, and after that, we'll worry about that."
"[I] thought more about it, but haven't made any decisions," manager Bobby Valentine said of the six-man idea. "We'll just see how these games go."
Cook said his throwing shoulder is in strong shape after the team felt in Spring Training that Cook would not be a viable relief option because of the shoulder.
"[It's] way better. I don't even know if you can put the shoulder from last year and the shoulder from this year in the same category," Cook said. "They have a really good shoulder program here. The training staff has done an excellent job monitoring everything from Day 1 of Spring Training, and it's really paid off being patient.
"My consistency is what's better. I'm not throwing much harder than I was, and I have the confidence to go out there and make pitches knowing that I'm able to just throw it and not be hurt. For a pitcher going out there, when you have confidence, that can lead to success."
Cook struck out 13 and walked 11 in 33 1/3 innings with Pawtucket.
The Sox made room for Cook on the 40-man roster by transferring Carl Crawford to the 60-day disabled list. Jose Iglesias was optioned to Triple-A Pawtucket to make room on the 25-man roster.