Speaking to reporters Sunday, Miller said he thinks "there's still some doubt that we're going to have a season now."
There have been reservations from multiple players about whether or not this season will happen.
San Francisco Giants catcher Buster Posey told The Athletic's Andrew Baggarly he is still unsure about playing the full schedule:
"I want to see how things progress here over the next couple weeks. It would be a little bit maybe naive or silly not to gauge what's going on around you, and not only around here but paying attention to what's happening in different parts of the country. It's obviously unprecedented times right now. Most definitely, I've thought about it and talked with my wife about it quite a bit."
MLB will reportedly open the 2020 season on July 23 with the New York Yankees vs. the Washington Nationals and San Francisco Giants vs. the Los Angeles Dodgers, per USA Today's Bob Nightengale.
The remaining 26 teams will play their first game the following day, with Nightengale noting the full schedule will be released Monday.
Players arrived in their team's city this weekend for the start of summer camp.
MLB and the MLB Players Association announced Friday that the first round of COVID-19 tests included 38 positives (31 players, seven staff members), with 19 different teams having at least one positive test.
Miller, who is entering his second season with the Cardinals, serves as a player representative on the MLBPA's executive board.