Youkilis was placed on the 15-day disabled list with a low back strain prior to Wednesday night's game against the Oakland Athletics. That paved the way for Middlebrooks to bat eighth in manager Bobby Valentine's lineup.
In his Major League debut, Middlebrooks looked worthy of the hype, contributing with an infield single, an opposite-field double and a stolen base during Boston's 4-2 loss.
"It was unbelievable," Middlebrooks said. "Words can't describe it. Just coming here and playing in Fenway Park and with these guys, it was unreal."
The 23-year-old Middlebrooks is ranked No. 1 on MLB.com's list of Top 20 Red Sox prospects. Selected in the fifth round of the 2007 First-Year Player Draft, the 23-year-old was hitting .333 with nine home runs and 27 RBIs in 24 games and 93 at-bats with Triple-A Pawtucket.
"You know, he's been hot in Triple-A and played very well in Spring Training," said Valentine. "He definitely adds a little interest and hopefully gives us a little something we can look forward to."
As for Youkilis, Wednesday marked the fourth straight day he wasn't healthy enough to play. His move to the DL is retroactive to Sunday, meaning he will be eligible to return on May 14.
Youkilis is optimistic he'll be ready by then. He feels he's made good progress over the last 24 hours with Dan Dyrek, the team's back specialist.
"I've just had an irritated joint or whatever in the back that has been happening for a little bit of time now," Youkilis said. "Basically, what we've done is, the last couple of days have been bad and we've tried to get through it, and we've made great strides from yesterday to today.
"Things are going well, but they just wanted to get a step ahead of it and try to get me as healthy as possible. Working with Dan, he's told me that this is good just to get some time and basically to get in there and dig in and this will help you out through the whole year."
It has been a tough start for Youkilis, who was bothered by a groin issue a couple of weeks ago and is hitting .219 with two homers and nine RBIs. Youkilis missed a large chunk of time in August and September last season and underwent sports hernia surgery.
"He was starting to feel better and swing the bat better, and there was some good signs through the end of the homestand, early part of the road trip, and he started to put some numbers together and then the back started bothering him over the weekend in Chicago," said general manager Ben Cherington. "He's just got to feel better physically. Coming off last year where he missed a lot of time, he was just kind of getting back in [rhythm].
"It was an unorthodox approach -- obviously -- with what he does with his hands and he was starting to get comfortable with that again when the back flared up. We just have to get him feeling better. If he feels better and is playing consistently, we know he's going to hit. We have to get to that point."
Youkilis is obviously frustrated by his latest injury, but he is keeping a positive attitude.
"It's frustrating," Youkilis said. "I don't want to be injured. I want to play. There's nothing I can do but get healthy and try to help this team win."
At least for the short term, Middlebrooks will try to help the Red Sox win.
Did Valentine have any advice for Middlebrooks?
"Have fun. Enjoy it," Valentine said. "Get into the moment. You know, he's good at that from what I saw in Spring Training."
Middlebrooks had been waiting for this day for a long time.
"About 23 years. It feels great," Middlebrooks said.
They only thing that could have made the day feel better for Middlebrooks would have been a win.
"I wish we could be in there all celebrating, sharing the delight that he'd really have if this was a victory," Valentine said. "But it was a terrific game he played, nice defense, the walk, the hustle, the stolen base, he was one of the right-handed hitters to go with that cutter to right field. He did a really good job."