SEATTLE — One-time top Yankees pitching prospect Justus Sheffield will get his day in the spotlight against the Yankees come Wednesday.

The lefty is excited to see some of his old friends and perhaps offer the Yankees a glimpse of what could have been. But trades are a way of life in Major League Baseball, especially for pitching prospects, and Sheffield was part of the deal that enabled the Yankees to acquire James Paxton, who is pitching like the Yankees’ top starter for the postseason.

Sheffield will match up against Paxton in a battle of lefties.

“It’s been a pretty weird year for me,’’ Sheffield told The Post on Monday at T-Mobile Park. “Not starting off too hot and the demotion and everything like that, but I just wanted to keep my head and get better personally every day no matter where I was.’’

That mature approach has paid off.

Sheffield was a hard worker for the Yankees. The only question is: Does he have enough stuff to offer true starting success in the majors?

Some scouts believe his stuff falls a little short.

“It’s going to be fun facing them, knowing some guys over there,’’ Sheffield said. “There will be some familiar faces, but it’s the same game no matter who is in the other dugout.’’

Sheffield wound up getting bounced all the way down to Double-A Arkansas from Triple-A Tacoma this season. He was 2-6 at Tacoma with a 6.87 ERA, but remember the major league ball is in play in Triple-A and it really flies in the PCL. At Arkansas, Sheffield posted a 2.19 ERA over 12 starts. That gave the Mariners reason for hope, and he was called up Friday.

This will be his second major league start since being called up. He started against the Blue Jays in a 7-4 Mariners win Friday, going four innings, surrendering seven hits and three earned runs.

Progress is being made as Sheffield has made a significant step, throwing 142 ²/₃ innings this season, the most in his career.

Sheffield is only 23. He came to the Yankees in the trade with the Indians that also brought Clint Frazier to New York in the deal for Andrew Miller.

“I feel really good right now, and I know my innings are up higher than they’ve ever been, but I’m still feeling strong,’’ Sheffield said.

At Arkansas, Sheffield played with a young outfielder you might have heard of — one-time Mets top prospect Jarred Kelenic, who is only 19 and is on track to become a star, according to scouts.

“He’s really good,’’ Sheffield said. “He’s a gamer. He’s going to be really, really good in the future. Good kid, works really hard, has a plan.’’

That’s what it takes.

“With the Yankees I did everything I could personally do — just get better and pitch my butt off,’’ Sheffield said. “I fell into a trade, and with this game you never know where you are going to end up.’’

Sheffield has a fan in another lefty: CC Sabathia.

“I like his mentality and makeup, he’s a good kid,’’ Sabathia said. “I always root for him. Anything comes up I talk to him and help him.’’

Instead of pitching for the Yankees, Sheffield will be pitching against them and the player he was traded for, so this will be one interesting Wednesday.

Being with the Mariners, Sheffield said, “is a great opportunity. I just have to make the most of it.’’

His biggest challenge, Sheffield says, is something that only comes with experience.

“Really, just slowing things down,’’ Sheffield said. “Just keeping it simple, trusting my stuff, that’s pretty much it.’’

So what kind of young pitcher are Yankee fans going to see when they watch the game?

“They’re going to see a competitor,’’ Sheffield said. “Somebody that is really going to go out there and compete, you know just trying to do my job. Pitch as deep into the game as possible and keep the game as close as possible. Give my team a chance to win.’’

His team is now the Mariners.