On the second pitch he saw from Ricky Nolasco, Reyes lifted a home run to right field to highlight a workout at Roger Dean Stadium.
"I said yesterday, when we had the question-and-answer [period], 'Jose, hit a leadoff home run, we win a 1-0 game,'" Guillen said.
Laughing at his prediction, he added: "He did it the wrong day, though."
Meaningless or not, Reyes provided some excitement in what could have been a go-through-the-motions three innings.
Initially, the Marlins had been set to scrimmage on Field 2, but a heavy downpour an hour earlier prompted them to go inside the main ballpark, which was in better shape because of the infield tarp.
When Reyes stepped up in the bottom of the first inning, he was just trying to make something happen.
"The home run, you don't look for that," Reyes said. "I look to make solid contact."
Nolasco, the Marlins' No. 3 starter, shrugged off being victimized by one of the top leadoff hitters in the game.
"I wanted to make him feel good about himself, so he can help us," Nolasco quipped. "No, the sinker cut over the middle of the plate. [I need to] get them [homers] out of the way now."
Any time he hits the ball over the wall, Reyes will take it. But he noted that beginning on Monday, there will be more than a handful of media and Marlins front-office personnel sitting in the stands as the team opens its Grapefruit League schedule against the Cardinals at Roger Dean Stadium.
And after nearly two weeks of conditioning, fielding grounders and taking batting practice, the former Met is eager to start playing some games for his new team.
"This game doesn't count," the four-time All-Star shortstop said. "Tomorrow we'll play a real game. I feel good. I can't wait to get on the field and to play a real game. I've worked hard the last two weeks. I'm ready to go."
Josh Johnson will start for Miami. St. Louis is going with Kyle Lohse.
"It will be very exciting to get on the field with my new team," Reyes said. "We've been working together for almost two weeks. It will be fun to get on the field, and get to know how we work together on the field. I'm very excited right now, I can't wait."
In an offseason of unprecedented spending, Reyes was the Marlins' signature signing. His six-year, $106 million contract is the richest in club history.
With Reyes leading off, followed by Emilio Bonifacio and Hanley Ramirez, the Marlins will have what may be the fastest top three in the game.
"I think it's going to be great, because our lineup is going to have a lot of speed and a lot of power," Reyes said. "When you put together that combination, you are going to be dangerous.
"My job here is to going to be to get on base and score some runs. If I can do that the way I did in the past, I think we're going to score a lot of runs."
The energy and excitement Reyes brings was evident on Sunday, even in a low-key scrimmage.
"It's nice to see," Guillen said. "You [make] that money, and you're that type of ballplayer, and you go out there and play a 'B' game. That's something that is very positive for me and my coaching staff to see. To see all the players playing together, all nine guys going out there, ready to play."
To Reyes it didn't matter if Sunday was merely a structured tuneup on a soggy field. As long as he was on the field, he was going to have fun.
"If everybody is there, I'm there, too," Reyes said. "I'm a team guy. I like to be on the field with my teammates and enjoy the game."