The Yankees agreed to acquire J.A. Happ on July 26 because of Aug. 17.

It doesn’t have to be Aug. 17. It can be Aug. 6 or Sept. 9 or really any day the rest of the season.

That would be the day CC Sabathia’s knee is too painful to allow him to pitch or Masahiro Tanaka’s elbow misery arises anew or … fill in the pitcher and the malady.

It would be one thing if Jacob deGrom or Madison Bumgarner was available to the Yankees. That would be the kind of top-of-the-rotation, start-Game-1-or-2-of-a-playoff-series hurler the Yankees would be moving heaven, earth and Justus Sheffield for.

But in agreeing to turn Brandon Drury and Billy McKinney into Happ, the Yankees were showing a willingness to pay a price for depth and protection in their rotation. Drury might have been relegated to a backup or the minors this year by Miguel Andujar. However, the Yankees obtained him believing he would be an above-average starting third baseman, and they still think that.

They gave up that — and Drury is not even arbitration eligible until after this season — plus McKinney, who may make a power-hitting fourth outfielder, for a starter currently not pitching well who will be a free agent after the season.

That the Yankees were willing to cede such value goes back to the symbolism of Aug. 17. The starters available now probably won’t be around Aug. 17 — most, if not all, traded by Tuesday’s non-waiver deadline. Trades can be made in August, but players have to pass through waivers, and the Yankees couldn’t know for sure that any would reach them, and even if they did, if they would be of any value.

So, the Yankees’ pressure to act now on Happ reflects concern about taking a hit to the rotation beyond the July 31 deadline and having to turn internally to a Sheffield or Chance Adams and hope for the best.

Instead, they will hope they get the best version of Happ. He was 10-3 with a 3.62 ERA through June before going 0-3 with a 7.41 ERA in four July starts.

In addition, Happ had a poor defensive team behind him with Toronto, plus the Yankees could protect him earlier in the game with their deep, talented bullpen. The other fringe benefit — as with fellow acquisition Zach Britton — is that he is a lefty. The Red Sox are weaker against lefties than righties, and New York and Boston have 10 games left. Happ is currently scheduled to start Saturday for Toronto, which, if he stayed in that spot, would line him up to pitch the following weekend at Fenway.

Happ has pitched twice against the Red Sox in 2018. On April 24, Happ allowed one run in seven innings, striking out 10 and walking none. On July 12, he gave up five runs in 3 2/3 innings, throwing 102 pitches. He did strike out six.

Only Bartolo Colon throws a higher percentage of fastballs than Happ (73.9 percent) among qualified starters. So, perhaps the Yankees, who believe in using breaking balls more than ever, will try to get the 35-year-old lefty to further incorporate his slider. That Happ has experience in the AL East is also beneficial.

But part of the Yankees’ hesitancy is that they might just have traded two more future pieces for a rental at a time when their most obvious road to the playoffs is as a wild card. Thus, they could be one and done and out before even using Happ.

In addition, they cannot be sure that even if they advance to the division series that Happ would start — though if it is against Boston, the chances rise. Luis Severino, assuming he is just going through a blip of ineffectiveness now, will start. So almost certainly will Tanaka and probably Sabathia, both owing to their experience and how well they performed in the playoffs last year.

But the Yankees are in a small window in which Sonny Gray and even Luis Cessa are pitching well.

The risk, though, is if that stops or someone gets hurt for a team that already lost Jordan Montgomery (Tommy John surgery) for the year. The risk is waking up on the symbolic Aug. 17 with a problem and no way to solve it.

So, the Yankees acted now and made the painful choice in particular to give up Drury as a way to add depth and potentially avoid a nightmare.