This year marks the 40th anniversary of the designated hitter. But hold the celebrations.

Because having the two leagues playing under different rules never has been more problematic than now. It is to such a degree that one veteran general manager said, “It is just a matter of time until the leagues are forced to play under one set of rules and, let’s face it, the DH is not going away, so that is going to be what everyone uses.”

Here are just a few of the main problems:

1. More teams can spend big money than ever. Yet, NL clubs are at a disadvantage to sign position players — specifically corner players — to the kind of mega-deals like Prince Fielder (nine years, $214 million with Tigers) and Albert Pujols (10 years, $240 million with Angels) did to switch leagues. NL teams have no soft-DH landing spot for such players to retain some value, even if they no longer can play the field competently.

There are nine current position players signed for at least eight years and $150 million. Six (Alex Rodriguez, Pujols, Fielder, Joe Mauer, Mark Teixeira and Miguel Cabrera) are in the AL. Of the three NL players (Joey Votto, Matt Kemp and Troy Tulowitizki), only Votto is a corner player, and all three extended with their teams before to free agency — just Cabrera and Mauer fit that category with the AL players.

“I think the DH rule is completely unfair to NL teams when it comes to free agency,” said an NL GM who has worked in both leagues. “An AL team can give a player an extra year or two that an NL team simply cannot. We are fishing in the same pool for players, but without equal ability to sign them.”

Perhaps it’s a blessing for NL teams to avoid these usually detrimental mega-deals.

“I’m sure the Yankees thought eventually A-Rod could DH — and maybe he still can,” an AL GM said. “But is that a good thing for them?”

It also could be that elite pitchers will gravitate to the NL as a counterbalance — though we have yet to see a strong migration pattern.

2. Houston’s shift to the AL removed the unfairness of having different-size divisions. Statistically, it was easier to win a four-team division like the AL West than a six-team division like the NL Central.

Fifteen teams in each league means six five-team divisions. Nevertheless, it also requires an interleague game daily.

Thus, instead of having 15-18 interleague games played in two clusters before the All-Star break, interleague games will be interspersed from the first to last day of the season.

So each AL club will have to decide how to prepare its pitchers to do something foreign — hit and run the bases — at different intervals rather than just twice all year.

Therefore, at a time when the sport is exploring ways to make the game safer — such as better helmets for hitters, perhaps a skull cap for pitchers and maybe the elimination of catchers blocking the plate — it is promoting a greater possibility of its most precious commodity, pitchers (specifically AL pitchers), getting injured.

“Any time you are having any player do something they don’t regularly do, you increase the probability of injury,” an AL personnel head said.

I know, I know, they are athletes and man up and all of that. But, in general, AL pitchers also have not played offense in the minors — and perhaps not as amateurs, as well. We saw Chien-Ming Wang’s career essentially ruined running the bases, and now pitchers not used to such activities are going to have to be ready to do it throughout the season.

“The issue of [AL] pitchers running and hitting and getting hurt is a real one,” said Michael Weiner, head of the Players Association. “Teams are going to have to be more careful. No one pretends the need for an interleague game every day is a positive consequence of [moving Houston to the AL]. No one wants [interleague games] throughout the year. So [an increase in AL pitcher injuries] is a real possibility.”

3. Each team will play 20 interleague games and no club will play more than one interleague series in September. Still, the most precious games on the schedule will be impacted. Consider the Tigers had no idea this was coming when they signed Victor Martinez to a four-year contract. Now, they end their season with three games at Miami. What if they need to win to get in the playoffs and can’t start one of their best and most expensive players?

For all of these reasons, more than ever, both leagues need to play under the same rules. Several executives suggested these problems are resonating with NL GMs who would want to see the rules changed. But ownership is more unwilling to give up the tradition of pitchers hitting.

But pragmatically there is just about no way the union is going to allow the DH — a well-compensated job generally given to veteran members of the association — to be bargained out. So it is time to adopt the DH for all games.

When Major League Rule 6.10 went into play — with Ron Blomberg acting as the first DH and hitting against Luis Tiant on April 6, 1973 — it was a reaction by AL clubs who equated declining offense with declining attendance. Nine of 12 AL teams drew fewer than a million fans in 1972. So they added an extra hitter to the lineup and removed the pitcher.

Attendance is not a major issue now. But after about two decades when it seemed every change — steroids, maple bats, smaller parks, etc — favored offense, the recent trends — computer-generated defensive alignments, more-detailed hot-and-cold zones to attack hitters and the ubiquity of the cutter — have advantaged the pitcher.

So making the pitchers face an extra hitter in all games would be an NFL-like decision to alter rules to favor offense. And, let’s face it, with substitutions on nearly every play, the NFL has prepared a generation of sports fans to expect and respect specialists.

I know some “purists” will go bonkers. But when they say they want the game the way it used to be, do they mean pre-1920, when spitballs were OK, or pre-1947, when no players of color were allowed? The game always is evolving.

Weiner said altering the DH dichotomy did not come up in the last round of collective bargaining. But I wonder what happens if the best free agent hitters continue a march in one direction or AL starting pitchers begin going down more regularly with injuries from hitting or running the bases.

joel.sherman@nypost.com