Thanks to a contract holdout (2009), a neck injury (2010), foot surgery (2011) and an Achilles tendon tear (2013), wide receiver Michael Crabtree hasn't played in a preseason game in four of his first five seasons. Crabtree will probably miss the preseason opener Thursday in Baltimore because of an leg injury he suffered in practice this week, reportedly a strained hamstring, according to a source of ESPN's Josina Anderson. The good news for the 49ers is that Harbaugh said Crabtree's injury isn't expected to sideline him for more than two weeks, and the receiver didn't aggravate the Achilles tendon injury he suffered last year or reinjure his twice-broken left foot. The 2013 fourth-round pick remains on the non-football injury list because of both his knee and an offseason hamstring injury. Harbaugh sent Anquan Boldin to the locker room with about five minutes left in practice after the wide receiver scuffled with cornerback Darryl Morris following a catch over the middle. Earlier, Harbaugh got in the middle to help break up a shoving scrap between outside linebacker Chase Thomas and running back Jewel Hampton.