NEW YORK -- Mets left-hander Johan Santana is not expected to pitch again this season, general manager Sandy Alderson said Wednesday.

An MRI taken Tuesday at the Hospital for Special Surgery in Manhattan revealed Santana has lower back inflammation. The 33-year-old left-hander will be placed on the 15-day disabled list retroactive to Aug. 18 but is expected to be shut down for the rest of the year.

Surgery will not be required, Alderson said. Doctors have prescribed rest and medication and intermittent exercise.

"I'm very confident he'll be back next season ready to go, hopefully in a stronger position coming into this season," Alderson said. "If you look back at this season and what we reasonably could've expected at the beginning of the year, he's accomplished quite a lot."

Santana said he began experiencing tightness in his back a couple weeks ago, but it wasn't until his last start Aug. 17 in Washington that he really felt it. He brought up the issue with the organization following a Monday bullpen session.

"My mindset was to start tomorrow, but the doctors are thinking different now, so we'll go with everything they're saying," said Santana, who is due $31 million next year, including the buyout for the following season. "I want to keep pitching. I felt that I could pitch, but at the same time, I'm listening to them."

Alderson is relieved Santana won't need surgery.

"I don't believe it's a disk; it's inflammation in the area of [vertebra] L5," Alderson said, adding there's no concern with Santana's shoulder or ankle. "That inflammation has led to some soreness, but again, I think the important thing is, that the doctors have not recommended surgery."

Right-hander Collin McHugh was promoted from the minors and will start in Santana's place on Thursday afternoon against Colorado. McHugh, 25, went 2-4 with a 3.39 ERA in 12 starts for Triple-A Buffalo.

Since throwing a career-high 134-pitch no-hitter -- the first one in franchise history -- on June 1, Santana is 3-7 with an 8.27 ERA in 10 starts. He has allowed at least six runs in each of his last five outings (0-5, 15.63 ERA).

"I don't have any second thoughts about the way it was handled," Alderson said of leaving Santana in the no-hitter instead of pulling him. "Obviously, it's something that comes up. I think the reason I don't spend a lot of time thinking about it is there's nothing I can do about it. And No. 2, however, is that this is a back issue that was preceded by an ankle issue. Neither of which pre-existed at the time of the no-hitter. This has not been a shoulder issue, so from that standpoint I just don't see a direct correlation.

"As good an explanation as any is that this is a substantial number of innings that he pitched over the course of the season following a season of no activity other than rehab," Alderson continued. "This may just be a way that his body is telling him that it's been a long time since the injury, through the rehabilitation and the number of innings he's pitched this year, and the body is starting to complain. But again, I think it would be very hard to trace it back to the no-hitter."