Baseball's rule against the use of foreign substances has been buried for decades underneath a gentlemen's agreement held among managers, who almost uniformly refused to ask umpires to check opposing pitchers because they knew that their own pitchers would not be checked.

But with Major League Baseball set to order umpires to enforce the foreign substance rules starting Monday, at least three teams intend to set aside that old agreement, according to sources. If the managers of those teams receive information that seems suspicious -- video capturing an opposing pitcher perhaps using foreign substances, or data about an unusual spike in spin rate -- they will ask umpires to check opposing pitchers.

If just a handful of teams start to ask for foreign-substance checks, the gentlemen's agreement could become obsolete, with most or all teams willing to monitor opposing pitchers.

A high-ranking talent evaluator explained his organization's perspective the other day, while asking not to be identified.

"We've been telling our pitchers that if they have been using, they need to stop," the evaluator said. "We are looking to level the playing field. The whole sport is looking to level the playing field. We have an expectation our guys will honor the rule. If we get an indication that there's someone pitching for the other team who might be doing something to gain a competitive advantage, yes, we will want our manager to challenge that."

Staffers from two other organizations echoed similar sentiments -- that so long as there is a sportwide devotion to the level playing field and the elimination of foreign substances, they will expect opponents to live up to the rule as well.

The use of foreign substances has been widespread and essentially out in the open for many pitchers across the baseball landscape. When pitchers gripped the shiny spot on the forearm of their gloved hands, just about everyone in the sport -- umpires, managers, coaches, players -- was aware that they probably were accessing some mixture of sunscreen, rosin and pine tar, which is against the long-standing rules.

But just about every manager has looked the other way rather than ask umpires to check, mostly to protect their own pitchers. There have been rare exceptions. In a game between the Yankees and Red Sox in 2014, on a cold early-season night in Fenway Park, Michael Pineda struggled to grip and command the baseball in the first inning -- and complained about this to teammates after the first half-inning. When he returned to the mound for the bottom of the second, he had a gob of pine tar on his neck that had been applied by a veteran teammate -- a lump of foreign substance so large that it immediately drew the cameras of a national television audience.

As the buzz in the ballpark grew, Red Sox manager John Farrell emerged from the Boston dugout to ask that Pineda be checked -- but as Farrell explained years later, he really had no desire to do this, but felt he had no choice given how egregious Pineda's use was. Joe Girardi, the Yankees' manager at the time, absolved Farrell, telling reporters he understood that Farrell was in a difficult position. The umpires ejected Pineda, who was subsequently suspended 10 games.