Jockey Patrick Valenzuela will undergo knee surgery in the next couple of days and is expected to be out of action some four to eight weeks. Valenzuela, who took the jockey title at the recently concluded Santa Anita Park meeting, was thrown off his mount, Mango Escapade, at the beginning of the eighth race Sunday at Hollywood Park.Valenzuela was taken to nearby Centinela Hospital in Inglewood with a concussion, and subsequently was found to have torn a knee tendon, according to his agent, Ronnie Ebanks. Valenzuela still has fluid in his head from the concussion, and is currently in the intensive care unit."Patrick is not sure what happened," Ebanks said. "His hands lost the mane as they left the gate and he fell backward. When he tried to come back up, he thinks he blew an iron."Valenzuela had just returned to California from Kentucky, where he rode Coin Silver to a 12th-place finish in the Kentucky Derby (gr. I).