"Rule changes, in the middle of a season, can be challenging," Clark said. "I think something of this magnitude can be difficult to change. I won't say impossible -- I would never say impossible. But it can be difficult to change midstream."
Clark was responding to comments that MLB executive Tony La Russa made to ESPN.com earlier this week, that baseball was searching for more "specificity" to clear up "ambiguities" in the collision rules. And La Russa suggested that the sport was "getting closer" to accomplishing that.
However, Clark told ESPN.com that the union is still in the process of gathering information from players on how they feel the new rule is working, particularly in terms of how effective it has been in "changing habits" of both catchers and baserunners on plays at the plate.
"What we're doing on our end is exactly what we told MLB we were going to do all along," Clark said, "which was to gather information from the guys who were willing to offer it.
"We will suggest that players continue to offer their input. And as they offer their input, we will continue to have discussions about what clarifications and/or adjustments may need to be made going forward."
MLB and the union recently came to an agreement that addressed issues involving a change in the interpretation of the transfer rule, which applies to players moving the baseball from their glove to their throwing hand after making a catch.
But an adjustment to the collision rule would be more difficult, Clark said, because it would involve a rule change. The discussions about the transfer rule, which were initiated by the union, simply centered on a new interpretation of a longstanding rule.
Clark said the union remains in "ongoing" conversations with MLB about the collision rule, and said those conversations would continue.
"As we have said all along, we will continue to monitor this issue and will discuss changes with MLB as the season progresses," Clark said.
When the new collision rule was agreed to this March, it was termed "experimental," and was implemented with the understanding that the two sides would revisit the wording of the rule following the season.
So the union's focus, Clark said, remains on "gathering information, ... (and) asking players, 'What have you seen?' ... 'What conversations have you had?' ... 'What have you experienced?' ... and the like. So we're doing that literally as we speak."