NEW YORK (AP) — Players' union head Tony Clark took the extraordinary step of saying baseball fans should question whether it makes sense to purchase tickets for some teams, responding to Commissioner Rob Manfred's assertion that free-agent players have failed to adjust their economic demands in a market upended by analytics.

Top free agents Bryce Harper, Manny Machado, Craig Kimbrel and Dallas Keuchel remain unsigned with spring training under way, creating tension during negotiations on management's proposals for a pitch clock and new limitations on relief pitchers. The union responded with a wider list of plans that include economic initiatives such as expanding the designated hitter to the National League and altering the amateur draft to make rebuilding less appealing.

"Markets change," Manfred said Sunday. "We've had a lot of change in the game. People think about players differently. They analyze players differently. They negotiate differently."

Clark led negotiations in 2016 for a five-year labor deal. Players have increasingly been outspoken about their unhappiness during a second straight slow free-agent market, one that has seen many veterans take significant pay cuts and others remain without deals.

"Players' eyes don't deceive them, nor do fans'," Clark said in a statement Monday. "As players report to spring training and see respected veterans and valued teammates on the sidelines, they are rightfully frustrated by a two-year attack on free agency. Players commit to compete every pitch of every at-bat, and every inning of every game. Yet we're operating in an environment in which an increasing number of clubs appear to be making little effort to improve their rosters, compete for a championship or justify the price of a ticket."

Average attendance has dropped for three straight seasons and last year fell below 30,000 per game for the first time since 2003.

Players rebuffed management's proposal for a pitch clock ahead of the 2017 and 2018 seasons. Management made the unilateral decision to experiment with a pitch clock during spring training. Manfred has said he is reticent to change regular-season playing rules unless players agree.

"Players have made a sincere attempt to engage with clubs on their proposals to improve pace of play and enhance the game's appeal to fans," Clark said. "At the same time, we have presented wide-ranging ideas that value substance over seconds and ensure the best players are on the field every day. We believe these substantive changes are imperative now — not in 2022 or 2025, but in 2019."

Players have become increasingly outspoken about the slow pace of agreements and have mentioned the possibility of a work stoppage in 2022.

"Everybody sees it. It's obviously not good for baseball," said Los Angeles Angels outfielder Mike Trout, a two-time AL MVP who can become a free agent after the 2020 season.

"You got two of the top guys not signed yet. With teams saying they want to rebuild, why not start with one of the top guys? Manny, Bryce, look at the pitchers out there. It's pretty incredible. It's disappointing for the players. It's good they're standing up for themselves."

The union is concerned that too many teams are rebuilding, trying to emulate the Houston Astros. Houston lost 106 to 111 games in three straight years from 2011-13, earned three straight No. 1 draft picks and won its first World Series title in 2017.

"This narrative that our teams aren't trying is just not supported by the facts," Manfred said. "Our teams are trying. Every single one of them wants to win. It may look a little different to outsiders because the game has changed, the way that people think about the game, the way that people think about putting a winning team together has changed, but that doesn't mean they're not trying."

Chicago Cubs Chairman Tom Ricketts defended his team's decision not to spend money on this offseason's most illustrious free agents.

"That's a pretty easy question to answer. We don't have any more," he said. "The fact is that we've been in the top five in spending, in baseball spending, for the last five or six years. We're in the top couple last year. We've put our money back on the field. Unfortunately, you just can't have a high-profile free agent every single year. And part of that is obviously is how much it costs, whatever, $25 million, $30 million it's going to cost, plus it's a 10-year commitment. ... As much as I would love to have a great, new, exciting player every single season, it just can't happen every year."

Chicago is projected to exceed the $206 million luxury tax threshold along with the World Series champion Boston Red Sox and the New York Yankees.

More players are considering long-term contracts with their current teams rather than going on the market when they can. Oakland's Khris Davis, the big league home run champion, is among them. He can be a free agent after this year's World Series.

"It's not a good thing to be a free agent right now," he said.