Michael Wilbon and Tony Kornheiser discuss the proposed pitch clock for major league baseball and whether it will help cut down on game times. (1:42)

Pace-of-game talks between Major League Baseball and the players' association have stalled, increasing the likelihood that baseball will unilaterally implement new rules in advance of the 2018 season, a source told ESPN on Thursday.

Players have expressed concerns about the addition of a 20-second pitch clock -- the central component of commissioner Rob Manfred's pace-of-game initiative. Barring a change in the tone of the discussions, it appears MLB will implement the clock and a restriction on catcher mound visits in 2018 without the union signing off on the changes.

"My preferred path is a negotiated agreement with the players,'' Manfred said during the quarterly owners meetings in Florida in November. "But if we can't get an agreement, we are going to have rule changes in 2018, one way or the other.''

In 2015, MLB placed timers at ballparks to minimize down time between innings and introduced a rule requiring hitters to keep one foot in the batter's box barring several exceptions. The changes reduced the average game time by six minutes, but MLB games returned to three hours on average in 2016 and spiked to a record 3:05.11 in 2017.

After averaging 22 seconds between pitches in 2017, MLB has proposed a 20-second pitch clock for the 2018 season to shorten games. Last season, MLB games took an average of 3 hours, 5 minutes to complete -- a record. AP Photo/Bill Wippert

MLB and the union have held talks throughout the past year and met in Washington, D.C., earlier this month. The union has since reached out to its membership and held a conference call among executive board members and team representatives Tuesday, during which players expressed opposition to MLB's proposal.

Subsequent talks haven't yielded progress.

While players agree that pace of game is a topic to be addressed, they've contended that games can be shortened through revisions in instant replay, stricter monitoring of the down time between innings and other remedies that don't require a clock, a source told ESPN.

The players even discussed the possibility of bringing back bullpen carts, though that initiative didn't gain much traction.

Baseball introduced a pitch clock in the minor leagues in 2015. The clock that the commissioner's office is now proposing would allow for 20 seconds between offerings for big leaguers -- or two seconds less than the average of 22 seconds between pitches in 2017.

If a pitcher commits a clock violation, a ball will be assessed to the count. If the hitter violates the time limit, a strike will be incurred.

Dan Halem, MLB's chief legal officer, said in November that MLB needed to complete discussions by mid-January to roll out changes and explain them effectively to umpires and clubs.

Any rules changes require the approval of MLB owners, who are scheduled to hold their quarterly meetings Feb. 1 in Los Angeles.