Pat Metheny's An Imaginary Day is morphing into a not-so-imaginary percussion concerto -- to be performed this week by the Philadelphia Orchestra at its Thursday-through-Saturday concerts.

The much-lauded jazz guitarist will not be on stage at the Kimmel Center. Imaginary Day: Concerto for Vibraphone, Marimba, and Orchestra is actually the brainchild of principal percussionist Chris Deviney, a Metheny admirer who has been eager to bring together the two musical worlds and the listeners who come with them.

"I think those audiences have more in common than they think they do," said Deviney, who built and orchestrated the concerto from three extended cuts ("The Awakening," "Across the Sky," and "The Heat of the Day") from the decorated jazz guitarist's 1997 album.

Listeners familiar with Metheny's sophisticated, rhythm-driven, heavily synthesized textures that freely mix Celtic and Spanish influences may find it hard to imagine where the meeting point will be.

"I'm literally taking the primary voices in the tunes and and shifting them between myself and my partner [marimba player] She-e Wu," Deviney said. "In one section, there's a long, extended piano solo. A pianist can play 10 notes at the same time. I can only play four [with two mallets in each hand]. So I just separated the hands [of the piano solo] into two players."

"Hear that?" he said the other day in the percussion room backstage at the Kimmel Center while listening to one of Metheny's originals, "this is where the brass come in. When I first heard it, it was so orchestral in nature."

Deviney also follows Metheny's lead into electronica, using a "MalletKAT MIDI controller" -- a synthesizer, in other words -- as well as a vibraphone whose bars come with a tiny fan underneath that produces a vibrato effect when hit by mallets. By rubbing the mallet along the bar, Deviney creates the pitch-bending effects Metheny so easily produces on guitar.

So respectful is Deviney of the Metheny originals that he says there will be no improvising.

That's what he thinks. Conductor Bramwell Tovey, a frequent guest who is conversant in a broad range of genres, will double as a pianist. "There's a part of the piece that's like one big jam session," Tovey said by phone from Chicago. "It's inevitable ... to get carried away in an improvisatory fashion. I like that side of my life."

The arbiter of such decisions would typically be the composer. But Metheny -- one of the most honored jazz musicians of our time, with 20 Grammy Awards in 12 categories -- has had minimal creative input. Due to performance-schedule conflicts, he and his composing partner Lyle Mays won't be on hand to hear any of the three Philadelphia concerts, said a spokesman from the orchestra. "There are some jazz greats who find it hard to get out of bed in the morning and work," says Tovey. "Metheny seems not to have that problem."

The 10-year gestation of the piece began when Deviney pitched the idea for the piece to Metheny after a concert at the Wilmington Grand Opera House. Deviney recalls Metheny's asking which three cuts from the album he wanted to adapt. "I told him. He thought for a second, looked up at the ceiling, and said, 'Yeah, I think those would work well.' "

The speed of that decision is remarkable, considering what a huge body of work Metheny has, and that their meeting happened nearly a decade after the Imaginary Day album.

You wouldn't peg Deviney, 51, for a jazz type, with his serious demeanor and eternally knitted eyebrows. Yet the percussion room at the Kimmel Center is decorated with whimsical souvenirs -- kites from China, for example -- that show he has another side, one that grew up outside Pensacola, Fla., being wowed by Pat Metheny records, which makes him a natural guide to a project with such obvious outreach potential for the orchestra.

Yet it kept getting waylaid. Then-music director Christoph Eschenbach gave the project a "yes" but resigned from the orchestra a short time later. Charles Dutoit's time as chief conductor was limited. While on a tour stop in Macau about two years ago, Yannick Nézet-Séguin asked for a meeting with Deviney, who was not anticipating a happy outcome. But the music director had studied the materials and said, "I think we're going to do it."

In disbelief, Deviney said, "Are you sure?"

When Tovey was tapped to conduct, Deviney said, "He's the guy."

Tovey has also programmed a jazz-influenced work from a previous era, Leonard Bernstein's Prelude, Fugue, and Riffs -- written for Woody Herman, premiered by Benny Goodman, and conducted by Tovey in 1986 with liberal coaching from Bernstein himself.

"Not every orchestra has an attitude of, 'Let's give this a go,' " says Tovey. "But these kinds of things happen in Philadelphia. The orchestra has a spirit of adventure and fun that makes stuff like this possible."

The Philadelphia Orchestra program is to be performed Thursday, Friday, and Saturday at the Kimmel Center. Tickets: $48-$152. Information: 215-893-1999 or www.philorch.org.