Ramos suffered the knee injury on May 12 in the seventh inning against the Reds, as he chased after a ball that had ricocheted off his glove on a pitch from Jordan Zimmermann.
In other injury news, right-hander Ryan Mattheus said his left foot is feeling better and he is now throwing on flat ground. Mattheus is not sure when he will start a rehab assignment, but Johnson said the team is going be cautious when it comes to Mattheus, who is on the DL because of a plantar fascia strain.
"It's still tender. The concern is that if he were to field a ground ball or made a turn -- it's his landing foot, not his push-off foot -- he could tear [the plantar fascia] even further, and that would not be a good thing," Johnson said. "We are probably going to be cautious with him. If you talk to Ryan, he will say, 'I'm ready to pitch.' The doctors are being conservative on it, and rightfully so."
Infielder/outfielder Mark DeRosa, who is on the disabled list because of a left oblique strain, worked out on Saturday at Nationals Park without any problems and will more than likely start a rehab assignment with Class A Potomac, according to Johnson.
DeRosa most likely get four at-bats and then be a candidate to return to the team. The team needs help off the bench after Chad Tracy went down with a right adductor strain.
Reliever Brad Lidge is looking to start a rehab assignment early next week for Class A Potomac and hopes to rejoin the team during the Red Sox series starting June 8, or the following three-game series against the Blue Jays.