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HOUSTON — The Mets’ Opening Day roster is complete. 

Instead of heading to the bullpen, Paul Blackburn will start the season on the 15-day injured list with right knee inflammation, president of baseball operations David Stearns said Wednesday. 

The injury occurred after Blackburn’s most recent spring training outing, when he reported soreness in the knee the two days following his appearance.

The right-hander received an injection in the knee and won’t throw for 7-10 days, but Stearns said he expects him back before the end of April. 

That means three potential pieces of the rotation will open the year on the IL, although there is optimism Sean Manaea (oblique) and Frankie Montas (lat) are headed in the right direction, with Manaea already back in a throwing program and a recent MRI exam of Montas’ injury showing “good healing,” meaning he could begin throwing soon. 

With Blackburn out, both Max Kranick and Huascar Brazobán were informed they would be on the Opening Day roster. 

And on the position player side, Luisangel Acuña got the final spot, a source confirmed. 

Initially, Stearns said they would wait until Thursday’s noon deadline to determine the last spot, but they informed Acuña he was on the roster. 

Acuña could potentially start at second base on Thursday against Houston lefty Framber Valdez. 

He impressed in his short stint in the majors in September when Francisco Lindor was out and he played well this spring. 

“He’s a very gifted defender,” Stearns said. “I’m glad he got some added exposure at third base. … Offensively, we saw some good signs. We also saw where there needs to be some growth and he’s very aware of that.” 

Brett Baty will likely play regularly at second in the absence of Jeff McNeil (oblique), whose injury is now asymptomatic. 

“He had a great spring,’’ Stearns said of Baty. “He played pretty well at second base.” 

“I earned that job last year, too,” Baty said of winning the third base competition a year ago out of spring training, only to lose it to Mark Vientos in May. “This will be my second Opening Day. I’m proud of myself and happy.” 

Hayden Senger’s feel-good story is nearly complete, as the catcher was informed he will be on the Opening Day roster, as well. 

Senger, who turns 28 next week, has played 402 minor league games, but took advantage of the opportunity presented by Francisco Alvarez’s fractured hamate bone that will knock him out in the early going. 

The former 24th-round pick out of Miami (Ohio) works in the offseason at Whole Foods, and when Carlos Mendoza informed him he won the job to backup Luis Torrens, Senger said the manager told him: “I know what you do in the offseason. I don’t know if you ever got a promotion there, but you got one here.” 

“He earned it,’’ Stearns said. “He’s worked incredibly hard his entire career. It is a good story, but he’s not getting the job because it’s a good story. He’s getting the job because we think he can help us win games. He’s an elite defensive catcher that pitchers love throwing to.” 

The Mets will honor Ed Kranepool with a uniform patch of his No. 7 jersey number this season, the team announced Wednesday. 

Kranepool, a member of the franchise’s Hall of Fame, died in September at 79. 

Kranepool’s family will be honored prior to the home opener at Citi Field on April 4. 

The longtime Met spent his entire 18-year major league career in Queens, from 1962-79. He was a member of the 1969 World Series-winning team and played more games for the franchise than anyone in history. 

Kranepool is also among the top 10 in hits, doubles and runs scored in Mets history.