"It kind of crept on me before I knew it was there," Sizemore said. "The wind was blowing pretty good. I hit [the wall] pretty hard, but it hit me so fast I wasn't able to brace. But it didn't hurt too bad. I'm sure I'll be a little sore tomorrow, but I was lucky to catch it and come out with no injuries."
Two innings later, Sizemore was lacing a one-out single to right and scoring to jump-start a two-run rally.
Then, in the fifth, Sizemore again banged one into right for a single and would later score.
In this ESPN-televised contest pitting the two teams that played in the World Series last year, Sizemore kept filling up the highlight reel. There was a double off the Green Monster in the sixth, followed by his third run of the day.
And in the seventh, one more web gem. This time Sizemore lunged into the gap in left-center to make a diving catch against Jon Jay.
"Obviously, a very good day," said Red Sox manager John Farrell. "Both sides of the ball. Very good defense. Fearless, as we saw. And as we've talked about, the timing at the plate -- be it against left-handed pitching and right-handed pitching. He puts very good swings on some pitches. And maybe it was a little bit of a flashback to how good Grady was for a number of years."
Or, in an even better scenario, perhaps it was a sign that Sizemore will still be able to produce those type of exploits once the regular season starts.
"I'm just happy to be healthy, be out there playing and be able to do those things," said Sizemore. "Hopefully I can build off of that, put these days together, back-to-back, and still feel good, stay healthy and stay at it."
In a two-way clinic, Sizemore proved definitively on Monday that the skills are still there.
However, the durability questions won't be answered until Sizemore can actually get into the grind of the regular season and demonstrate that he can play five games in a row, or six out of seven.
"When you consider an everyday player and the number of games we're going to ask an everyday player to play, we don't have that many games in Spring Training," said Farrell. "He's passing every physical test to date. He came out of today feeling fine, even with all the activity that he had. We continue on."
Even Sizemore admits there are hardly any guarantees that he'll be able to hold up as an everyday player.
"I don't know. I don't have that answer," Sizemore said. "I think it's definitely a possibility. I don't know if they're going to roll me out there 25 out of 25. But let's just get through tomorrow before we start talking about the season. I just want to keep focusing on Spring Training and getting better and better."
All Sizemore can do now is take daily stock of how he is feeling.
"I feel good. I haven't had one day where I was like, 'I don't know if I can play today.' Or, 'This might be too much.' I can't really say because I haven't pushed it yet, but the important thing is I feel good enough to keep going out there every day and just build off of that," Sizemore said.
For the Red Sox, the evaluation process is similar.
"The best thing I can say is that he's responding favorably to everything we've put him through," Farrell said.
Either way, the decision won't be easy for the Red Sox. They have a young player knocking on the door and perhaps ready for his chance in Jackie Bradley Jr.
Bradley doesn't have Sizemore's durability questions, but he doesn't have his track record either.
When Sizemore banged into the wall on Monday -- and banged into it hard -- it was fair for the Red Sox to hold their collective breath for a second. This is a player who last played in the Majors in 2011.
"I think you're leery of any player crashing into walls," said Farrell. "He plays instinctually, he plays full out. And to ask a player to be less than that, that's a difficult request. He knows one way to play. So far, so good."
The competition will continue Tuesday, when Bradley gets his turn to start in Tampa against the Yankees. The tentative plan for Wednesday is for Sizemore to play in a Minor League game and for Bradley to start that night against the Pirates. Sizemore will likely be in the lineup for Thursday night's home game against the Yankees.
In a perfect world, perhaps there would be a scenario in which Sizemore and Bradley could both be on the roster.
But the Red Sox already have a full bench with proven veterans David Ross, Jonny Gomes and Mike Carp slotted for the first three spots and a utility infielder needed to the final reserve slot.
"Well, there's going to be a number of factors that go into that," said Farrell. "Who else is on the roster? What's the secondary position, the other alternative in center field? We've talked about wanting to keep Shane Victorino in right field. And that's still our intent, to keep him in right field as much as possible. So again, with each passing day, we get more information and we start to look at different sets of combinations."
Meanwhile, Sizemore just relishes the opportunity to do his job again.
"It's just fun. It's just a lot of fun," Sizemore said. "It's a loose atmosphere. Everybody works hard and plays well together. I couldn't ask for a better situation to be a part of."