The busy St. Louis Cardinals have added a big bat for their pennant drive.

OAKLAND, Calif. — The busy St. Louis Cardinals have added a big bat for their pennant drive.

A person with knowledge of the deal says the Oakland Athletics have completed a trade to send outfielder Matt Holliday to the St. Louis Cardinals. The person spoke to The Associated Press on condition of anonymity Friday because an announcement had yet to be made.

The Athletics will receive a package of prospects, highlighted by third baseman Brett Wallace. Outfielder Shane Peterson and right-hander Clayton Mortensen also will join the A's organization in a deal first reported by ESPN.com.

The first-place Cardinals have been looking to add another hitter in the middle of the lineup to support slugger Albert Pujols. St. Louis already acquired utilityman Mark DeRosa from the Cleveland Indians late last month and then shortstop Julio Lugo in a Wednesday swap with the Boston Red Sox.

Holliday has struggled in his first season with the A's, batting .286 with 11 home runs and 54 RBIs.

Oakland traded for Holliday during the winter knowing he might only be with the club for half a season. General manager Billy Beane acknowledged as much at the time. The 29-year-old Holliday had his best game for Oakland at home on Monday, hitting two home runs with a grand slam as the A's rallied from 10 runs down to beat the Minnesota Twins 14-13.

Many scouts attended recent games in Oakland to watch Holliday, who spent his first five major league seasons with the Rockies.

Wallace could provide the A's some depth at third base in the absence of six-time Gold Glover Eric Chavez, who is recovering from season-ending back surgery last month that he hopes will help salvage his career. It was the second back surgery for Chavez — he had the same microdiscectomy procedure in a different spot — and his fifth operation overall since Sept. 5, 2007. He also had three shoulder operations.

The A's hope Chavez will be ready by spring training 2010.

Wallace has made a quick move through the Cardinals' farm system and was considered their top offensive prospect. A first-round draft pick last year chosen 13th overall, he was already at Triple-A Memphis. Whenever St. Louis needed help from the minors, Wallace's name came up, but manager Tony La Russa insisted he needed more time.

The 6-foot-2, 205-pound Wallace, a left-handed hitter who was born in the Bay Area, was batting .293 with six homers, 19 RBIs and 11 doubles in 62 games for Memphis after starting the year in Double-A.

He played in the All-Star Futures Game in St. Louis earlier this month.

The Cardinals have been without third baseman Troy Glaus, who is rehabilitating from January shoulder surgery.

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AP Sports Writer R.B. Fallstrom in St. Louis contributed to this report.