Rick Kern/WireImageAcclaimed singer/songwriter Leon Russell suffered a heart attack during the past week and is undergoing tests in advance of an upcoming operation, the Oklahoma Historical Society reports. The 74-year-old musician is in stable condition and his representatives “are optimistic” that he will make a complete recovery.

A message on Russell’s official website says that he will undergo surgery this week, and he has postponed 28 upcoming concerts through a September 18 show in Houston. Leon has more than 20 additional performances on his 2016 itinerary, running from a September 22 gig in Newberry, South Carolina, through a December 1 show in Bend, Oregon.

During the 1960s, Russell established himself as an in-demand session keyboardist as a part of the famed collective of studio musicians called the Wrecking Crew. He went on to collaborate with Joe Cocker and Delaney & Bonnie, and also launched a successful career as a songwriter and solo artist. Among the well-known songs Leon’s written are “Delta Lady,” “Superstar” “This Masquerade” and “A Song for You.” Russell also scored top 20 hits with his own recordings “Tight Rope” and “Lady Blue,” in 1972 and 1975, respectively.

In 2010, Russell collaborated with Elton John — who long has cited Leon as a major influence — on the acclaimed album The Union. The following year Russell was inducted into the Rock and Roll Hall of Fame.

Here are all of Russell’s postponed concerts:

7/20-21 — Key West, FL, Key West Theater
7/22-23 — Boca Raton, FL, Funky Biscuit
7/24 — Clearwater, FL, Capitol Theatre
7/25 — Melbourne, FL, Maxwell C. King Center for Performing Arts Studio Theatre
7/27 — Pensacola, FL, Vinyl Music Hall
7/28 — Baton Rouge, LA, Varsity Theatre
7/29 — Fort Smith, AR, Peacemaker Music Festival
8/1 — Salina, KS, Stiefel Theatre
8/2 — Topeka, KS, Topeka Performing Arts Center
8/3 — Sioux Falls, SD, Washington Pavilion Arts Center
8/5 — Kansas City, MO, Knuckleheads
8/6 — Lincoln, NE, Bourbon Theatre
8/7 — Sturgis, SD, Buffalo Chip
8/9 — Missoula, MT, The Wilma
8/13 — Morristown, MN, Big Wu Family Reunion XVI
8/14 — St. Louis, MO, Ready Room
8/26-27 — Cortland, OH, Avamar Foundation
8/29 — Ann Arbor, MI, The Ark
9/1 — Fort Wayne, IN, Foellinger Theatre
9/2 — Milwaukee, WI, Pabst Theater
9/3 — Peoria, IL, Illinois Blues & Heritage Music Festival
9/16 — Oklahoma City, OK, Oklahoma State Fair
9/17 — New Braunfels, TX, Gruene Hall
9/18 — Houston, TX, House of Blues

Copyright © 2016, ABC Radio. All rights reserved.