Health issues facing Mick Jagger, the 75-year-old lead singer for the Rolling Stones, appear to be more serious than first thought when news surfaced over the weekend that the band was postponing its upcoming No Filter U.S. tour.

CNN cites a source close to the band saying Jagger is set to undergo heart surgery this week to replace a valve in his heart.

Jagger tweeted about the postponement of the tour over the weekend, apologizing to fans. You can see his tweet below.

The band also issued a statement apologizing for the postponement, saying: “Mick has been advised by doctors that he cannot go on tour at this time, as he needs medical treatment. We apologize for any inconvenience this causes those who have tickets to shows but wish to reassure fans to hold onto these existing tickets, as they will be valid for the rescheduled dates, which will be announced shortly.”

Stones lead guitarist Keith Richards wrote in a tweet: “A big disappointment for everyone but things need to be taken care of and we will see you soon. Mick, we are always there for you!”