After struggling in his season debut Wednesday, setup man Ken Giles stressed to Ryne Sandberg that it was a mechanical issue that plagued him, the Phillies manager said. The 24-year-old righthander has been adamant that the back tightness he felt late in spring training no longer ails him.

"It could be early- season rust or problems. It could be cold weather. It's hard to tell," Sandberg said Thursday. "A power pitcher like him, we'll see, maybe it will take him a little bit longer to really get fine-tuned and to where he wants to be with the velocity and with the mechanics."

Giles topped out at 96.6 miles per hour, according to the

Pitch f/x pitch- tracking system, and averaged 95 m.p.h. Last season, when he allowed just six earned runs in 452/3 innings, his fastball registered as high as 101 m.p.h. and averaged 97.96.

More troubling than his diminished velocity, though, was his erratic command. Only 15 of his 31 pitches Wednesday were strikes. The Red Sox swung at just four of his 24 fastballs. (Last season he induced swings on more than half his fastballs.) The outing followed a shaky spring in which he issued 12 walks in 13 innings.

Walks, while not a problem during Giles' incredible four-month major-league stint last year, were at times a problem when he was in the minor leagues. This time last year he was in double-A Reading working on mechanics and tightening his slider.

"If he can get his mechanics worked out and he's throwing 95, 97 and a slider, he can still be effective if he's throwing strikes," Sandberg said. "That's what he was lacking [Wednesday] night."

O'Sullivan starting

Sean O'Sullivan will serve as the Phillies' first No. 5 starter of the season Sunday when they host the Nationals.

O'Sullivan, now with triple-A Lehigh Valley, is not on the Phillies' 40-man roster, so the team will make a roster move after the game Saturday to accommodate his addition. The 27-year-old righthander made three appearances (two starts) last season for the Phillies and allowed nine runs over 122/3 innings.

Washington ace Max Scherzer will start opposite O'Sullivan. It will be Scherzer's first career start at Citizens Bank Park.

Extra bases

Rightfielder Domonic Brown (Achilles tendinitis) began his rehab assignment Thursday and went 0 for 2 with two strikeouts at high-A Clearwater. He played the first five innings. . . . Righthander Chad Billingsley, working his way back from two elbow surgeries, is scheduled to pitch four innings, weather permitting, in his Friday rehab start with Lehigh Valley.