Fox Sports’ Ken Rosenthal reports via Twitter that St. Louis Cardinals first baseman Albert Pujols will be out four to six weeks with a “small fracture” in his left forearm, suffered during Sunday’s game at Busch Stadium when the Royals’ Wilson Betemit collided with him in a play at first base.

Pujols, after a very slow start, was tearing it up in June; in 63 at-bats he was hitting .317/.419/.778 with 8 HR and 14 RBI, including walkoff home runs in consecutive games against the Cubs. This raised his season averages to .279/.355/.500; he was tied for third in the NL with his teammate Lance Berkman with 17 home runs.

Berkman will likely take over first base while Pujols is out. The four-to-six week estimate could wind up being low. The Cubs’ Derrek Lee suffered a similar injury on an almost-identical collision with the Dodgers’ Rafael Furcal on April 25, 2006. Originally supposed to be out as long as Pujols, Lee missed two months, attempted to come back and then missed another six weeks, coming back in late August and finally shutting his season down. At the time, Lee was coming off a MVP-quality season in 2005; it took more than two years until his power returned to its previous level for his 2009 season.

We hope to have medical analysis of this injury from SB Nation’s medical expert, Dr. Ali Mohamadi, later today.