Baez, 28, singled off of Reds reliever Amir Garrett with the bases loaded in the bottom of the ninth inning, bringing home Patrick Wisdom with the winning run. As soon as he made contact on the pitch, he began yelling at Garrett and continued to taunt him on his way to first base.
Cameras showed Baez mouthing the words "one pitch" to Garrett several times.
Baez and Garrett have a history of bad blood, with Garrett getting suspended for five games earlier this season for beating his chest and yelling in the direction of Cubs first baseman Anthony Rizzo after striking him out. Baez jumped out of the dugout to defend Rizzo, leading to the benches clearing.
It was Garrett who received the brunt of the punishment for the incident.
The two players also argued with each other during a game in 2018.
Cubs manager David Ross was concerned that Baez would get suspended for his actions on Monday, especially as MLB discourages taunting.