ST. PETERSBURG -- It's official: Chris Tillman's rough outing on Sunday was his last of the 2009 season.

Orioles manager Dave Trembley announced prior to the series opener against the Rays at Tropicana Field on Monday that his 21-year-old rookie starter will be shut down for the final week of the season. It means Tillman will miss his final start, which likely would've come Friday, and left-hander Chris Waters will take his place.

Jason Berken, originally scheduled to start Thursday's series finale, will be given an extra day, and Waters will go on Thursday -- following Mark Hendrickson, Jeremy Guthrie and David Hernandez.

Tillman finished the season 2-5 with a 5.40 ERA in 12 starts for the Orioles. On Sunday, he gave up six runs -- the most he's yielded all season -- in just two innings -- by far his shortest outing of the year.

But even though his first season in the big leagues ended on a sour note, Tillman is relatively pleased with how it turned out.

"Obviously I'd rather go into the offseason [coming] off a good start, but looking back at my season, I'm happy with it," Tillman said. "I'm happy with the progress I've made with my changeup, my curveball ... my fastball location. I probably wouldn't have even had this opportunity if certain circumstances didn't come up -- [with] guys getting hurt -- so I'm very thankful for that, and I'm very happy I got the opportunity to pitch here."

Tillman also went 8-6 with a 2.70 ERA in 18 starts for Triple-A Norfolk. With the Minors and Majors combined, he cranked it up to 161 2/3 innings during his fourth professional season, surpassing his previous high of 135 2/3 frames each of the previous two years.

In five September starts, Tillman went 1-3 with a 7.30 ERA. But he insists he wasn't laboring down the stretch.

"I felt great, actually, compared to last year," the young right-hander said. "Last year, I was a little sore, fatigued. This year, I actually felt like I got stronger and stronger as the year went on."

Now, Tillman's focus will turn to getting stronger in the offseason and improving in the first inning, a frame that saw him give up an opposing batting average of .340.

As for Trembley's advice?

"Be more aggressive right from the get-go -- right from the very first inning," Baltimore's skipper said.

"I think he needs to get stronger. I don't think he's fully going to develop until he's 24, 25, but for a kid who came to Spring Training as a 20-year-old and pitched in the big leagues at 21, I think he made a lot of progress."

Tillman's replacement, the 29-year-old Waters, went 9-7 with a 4.49 ERA in 29 games (20 starts) for Norfolk and gave up six runs in 6 2/3 innings (8.10 ERA) through four relief appearances for Baltimore this season.

In 11 starts for the Orioles in '08, Waters went 3-5 with a 5.01 ERA.