Youkilis has been one of the finest and most popular Red Sox players in recent years, but the expectation is that he will eventually be replaced at Fenway Park's hot corner by Middlebrooks, who is considered the best position prospect in the organization.
The question is when the true changing of the guard will take place.
Right now, the arrangement is decidedly temporary. Youkilis, 33, has a cranky back and was placed on the disabled list, eligible to return on May 14.
Until then, the 23-year-old Middlebrooks will play third base for the Red Sox. And one of the players he will receive advice from is the team-first Youkilis.
"It's a good opportunity for him to come out and play at this level, and it's going to be an exciting time for him to go out there and eplay," Youkilis said before Middlebrooks started his first game Wednesday night. "He'll remember this for the rest of his life, so hopefully he goes out there and does well. I was telling him, 'Don't try to do too much. Understand that you've played this game your whole life. This is another level, but you can play at this level.'"
Middlebrooks took in that advice from Youkilis and several other teammates, but if you are human, it still takes a few innings for it to sink in.
"Well, the first two innings, I was still in La-La land, so about the third inning, I realized it was just like any other game, just more people watching," Middlebrooks said. "I was kind of able to settle in and get comfortable."
While the first game was a spectacle, Middlebrooks will have a chance to settle in a little bit starting Friday night, when the Red Sox open a three-game series against the Orioles.
"Oh, for sure, I want to try to get into a groove and help this team win some games," Middlebrooks said.
For the next 10 days or so, Middlebrooks will be like one of those movies that debuts on select screens in advance of the actual nationwide opening date.
The Middlebrooks era could start at Fenway Park for good next season. The Red Sox have some evaluating to do, not just with Middlebrooks, but with familiar stalwart Youkilis.
The guaranteed portion of the contract Youkilis signed prior to the 2009 season expires once this season ends. But the Red Sox do hold a $13 million option they can exercise for 2013, or the club could set Youkilis free for a $1 million buyout.
Youkilis has had a hard time staying on the field the last couple of seasons. Yet the Red Sox know the type of hitter he is capable of being when healthy.
There's one scenario in which Middlebrooks and Youkilis could both stay -- if designated hitter David Ortiz leaves as a free agent. However, that doesn't seem likely, given the way Big Papi is producing, not to mention his iconic status within the organization and the city.
Youkilis was once the corner-infield prospect on the rise, getting his first chance to contribute in 2004 when Bill Mueller had a balky knee. Once Mueller returned, Youkilis rotated between the bench and the Minors.
At the end of 2005, the Red Sox let Mueller and Kevin Millar go as free agents, and Youkilis took over at first base for five seasons before moving back across the diamond last year following the trade for Adrian Gonzalez.
All Youkilis and Middlebrooks can do is go about their business, and the Red Sox will make the evaluations and decisions.
While the development of Middlebrooks might not be finished, he looked the part of a Major Leaguer on Wednesday, contributing with an infield single, an opposite-field double and even a stolen base. This, while making all the plays at third.
"Definitely," Red Sox outfielder Marlon Byrd said about whether Middlebrooks looked ready for the stage. "He looked good out there. He looked like he belonged."
"He looked great," Gonzalez said. "It was nice to see him out there, nice to see him beat out that hit and show his legs a little bit, especially after that stolen base also. And then got that double the other way. He's been swinging the bat great down there in Triple-A."
The Red Sox drafted Middlebrooks out of high school back in 2007, and they have watched with pride as he has rocketed through the farm system. He possesses the type of power bat teams are always looking for, and also the athleticism to make him a dynamic player.
"To see where he came from a as high-school kid and a kid who was actually more of a football player in high school -- hadn't played a lot of baseball until now -- it's an exciting day for him, an exciting day for the organization and a big credit to our scouting and developers," Red Sox general manager Ben Cherington said.
"This is not a guy that was a first-round pick, this is not a guy that was atop everyone's Draft boards. He was a talented kid that has a great work ethic and is a smart kid that's athletic and worked really hard to improve in all aspects of his game consistently over his time in the Minor Leagues. He obviously got off to a great start this year. He's a big part of our future, and for right now, he's going to fill in for Kevin for a little while, and we're just excited to watch him play."
Middlebrooks will thoroughly soak in his first stay in Boston -- even if it will likely be temporary.
"Of course, this is everyone's goal -- to play here," Middlebrooks said. "So like I said, I want to do what I can to help this team win games while I'm here."