The Mets have five veterans in their starting rotation, but only four of them are known quantities. 

When spring training commences in Port St. Lucie, Fla., in 2 ½ weeks, much of the curiosity will revolve around the “wild card” of the group, Kodai Senga, the Japanese ace whom the Mets signed last month to a five-year contract worth $75 million. Senga, who will turn 30 on Monday, will join Justin Verlander, Max Scherzer, Jose Quintana and Carlos Carrasco to give the Mets a deep and experienced rotation.

In an effort to get a jump on understanding Senga, pitching coach Jeremy Hefner, bullpen coach Dom Chiti and director of pitching development Eric Jagers traveled to Seattle within the last two weeks to watch the right-hander throw and to begin building a rapport with him.

“[Senga] is an extremely hard worker and I think he is going to be able to handle it well,” Hefner told The Post. “It’s just going to be different for him. How we handle that and the communication that we have with him and making sure that he’s feeling good and keeping him healthy for the entire season so that we can have him healthy in the playoffs will be important.”

Senga was accustomed to pitching once a week on a regular day, as is the protocol in Japan, and throwing 135-140 pitches in each outing. He will now face less predictability as part of a five-man rotation in which the ceiling is generally 100 pitches.

“Obviously Max and Justin can go deep into games,” Hefner said. “Cookie [Carrasco] showed that at times last year and then there is a little bit of a wild card with Senga and moving to a five-day spot. He threw a lot in Japan, but he was throwing a lot of pitches. It’s one of those things that is not sustainable on a five-day [rotation], so how deep he goes, that is still to be determined.

“We can’t really predict how he is going to respond, we can only continue to build that relationship with him and how he’s feeling and what he needs.”

Manager Buck Showalter, with an eye on every detail, said the staff locker room at Citi Field — which was previously beside his office — will be moved closer to the clubhouse in part to ensure Senga’s interpreter can be nearer the pitcher’s locker.

“We may even get to the point where we get the interpreter a locker in the locker room, but I would like to stay away from that if I can,” Showalter said. “I am a big believer that players only should [dress] in the locker room. The more [Senga] gets to interact with his teammates, it will have a natural flow.”

The Mets have often signed Japanese pitchers over the last 25 years, beginning with Masato Yoshii and Hideo Nomo. Others have included Kazuhisa Ishii, Ryota Igarashi, Hisanori Takahashi, and most recently Daisuke Matszuka.

Hefner got a sense of Senga’s personality during the visit to Seattle.

“He is funny,” Hefner said. “He doesn’t speak any English, but he is sarcastic and he is a lot of fun to be around. He has got a lot of energy, always smiling and it was a lot of fun to be around him for a couple of days.”

Catcher Michael Perez, who appeared in six games for the Mets last season, has agreed to a minor league deal with the club. Perez will help provide organizational catching depth behind Tomas Nido, Omar Narvaez, and Francisco Alvarez.