Officially, Francisco Alvarez’s recent rehabilitation assignment was merely a step on his path back to the Mets, necessary after a broken bone in his hand cost him the opening weeks of the season.

Unofficially, however, his tour of the Mets’ farm system provided an insightful, first-hand look at a burgeoning pitching pipeline, waves of talented, high-end arms that left the major-league team’s starting catcher excited about the franchise’s future.

Alvarez was behind the plate for a bunch of the pitchers well regarded by the organization, from flamethrower Raimon Gomez to upper-minors standout Nolan McLean, with plenty in between.

“It really surprised me when I went to the minor leagues and I saw the pitching staff — very strong. We’re doing a really good job,” Alvarez told Newsday on Wednesday. “When I was playing in the minor leagues, we didn’t have that talent. We have a lot of talented guys over there, a lot of young guys with good stuff.

“They’re going to help us maybe this year, maybe next year. At some point, they’re going to help the team a lot.”

Developing pitchers has been a focus area in recent years for the Mets, who haven’t had a homegrown true blue-chip prospect debut since Noah Syndergaard and Steven Matz in 2015 — yes, already a decade ago. Those efforts are starting to trickle up. As manager Carlos Mendoza said during spring training, “There’s a lot more coming.”

Alvarez caught glimpses of that. Consider: On his first day back in action with Low-A St. Lucie, his first batterymate was Gomez, a righthander who recently made headlines by throwing a fastball at 104.5 mph.

On this day, his return after missing nearly two years following Tommy John surgery, Gomez averaged a mere 98 with his heater. The first three in the first inning were 99.1, 99.4 and 99.8. He maxed out at 100.5 — rounded up to 101 on the scoreboard, naturally.

Welcome back to catching, Francisco Alvarez. Hope that previously broken receiving hand felt all right.

“He throws hard. He’s unbelievable,” Alvarez said. “One of my first batters, I didn’t catch a couple of fastballs. I was like, oh, [expletive[. Then 101. I saw the ball like [expletive].”

Gomez has 11 strikeouts and six walks in 7 2/3 innings (1.17 ERA) with St. Lucie. He could rise quickly if he throws strikes.

“The difference with the velo on his slider is big, so when he throws that fastball and throws that slider, it’s a big difference [and therefore effective],” Alvarez said. “That matters. But for me, it’s the velo — he’s throwing 97-plus every time.”

A few days later, Alvarez caught righthander Edgar Moreta, another breakout candidate. He doesn’t throw as hard — 94 mph on average with Alvarez — but has six separate pitches, as registered by MLB’s pitch-tracking technology. That is a lot for a 21-year-old. Through four appearances, he had 13 strikeouts and zero walks in 12 2/3 innings (4.97 ERA).

“Moreta has good stuff too,” Alvarez said. “He’s young.”

Upon joining Double-A Binghamton, Alvarez was behind the plate for a couple of closer-to-the-majors arms: righthanders Jonah Tong and McLean.

Alvarez liked Tong — whose 6-foot-1 frame and over-the-top delivery makes his mid-90s-and-sometimes-better fastball sneaky tough — even though that game wasn’t his best: five innings, three runs.

“That day he was having a tough day but he still threw five innings. That’s quality,” Alvarez said. “I said to the pitching coach, ‘He’s looking good.’ He’s like, ‘That was the worst he’s been.’”

Tong, who is well ahead of a normal age/development progression by reaching the upper minors at 21, struck out 19 batters in 8 1/3 innings in a pair of outings last week.

“He can locate the pitches, he can hit the corners, he’s throwing a little harder,” Alvarez said.

And then there was McLean, a former two-way player less than a year into focusing exclusively on pitching. Alvarez saw McLean at about his best: six innings, two hits, no runs, eight strikeouts (albeit with four walks).

“He can pitch,” Alvarez said, using a seemingly straightforward sentence as a strong compliment, especially for a young hurler. “Every pitch is plus. Sometimes I see a pitch and it’s in between. But with him, every pitch is working. That day, everything was working. It’s comfortable to catch when you have a pitcher like that.”

Mix in a few interesting others, such as Joander Suarez, plus the Triple-A names Alvarez had in spring training, Brandon Sproat and Blade Tidwell, and his conclusion was clear: The pitchers are coming.

“We have talent,” Alvarez said. “Those guys are going to make my job easier when they come here.”

The Mets put Danny Young on the injured list with a left elbow sprain, the severity of which they did not disclose. That left them with zero lefthanders in the bullpen. They called up righthander Chris Devenski from Triple-A Syracuse to take his place . . . Reliever Kevin Herget was returned to Syracuse after his two-inning cameo Tuesday . . . Paul Blackburn (right knee inflammation) is due to resume his rehab assignment with High-A Brooklyn on Friday . . . Brandon Nimmo (flu) was out of the lineup for a second game in a row but was feeling much better.