The Yankees may have just watched six days vanish that could have counted toward putting Kevin Youkilis on the disabled list. Now they are waiting on the results of an MRI exam today on Youkilis’ back to know if the corner infielder has to be shelved. Youkilis played in Saturday’s win over the Blue Jays after missing the previous six games due to lower back tightness. It was the Yankees’ hope that Youkilis was over the back problem. But he wasn’t in the lineup for yesterday’s 3-2 win over Toronto and knuckleballer R.A. Dickey, and manager Joe Girardi is worried. “It’s a little concerning for me because I have said all along, backs can be tricky,’’ Girardi said. “Sometimes you have no idea why it acts up. It’s concerning.’’ Asked if there was a chance Youkilis’ back troubles could develop into a long-range problem, Girardi crossed his fingers. “I sure hope that we aren’t where we were when he first did it,’’ Girardi said of Youkilis, who was removed from a game in Toronto on April 20 and missed the following six games. “But the only thing that is going to tell us is time.’’ Helping the Yankees be patient before dumping Youkilis onto a very crowded DL is that Corban Joseph, a second baseman who struggled defensively at third base during spring training, is the only minor league infielder on the 40-man roster. Infielders Gil Velazquez and David Adams are at Triple-A Scranton/Wilkes-Barre, but they aren’t on the 40-man roster. And there isn’t much flexibility to place a player on the 60-day DL. Francisco Cervelli, Curtis Granderson, Ivan Nova and Mark Teixeira are on the 15-day DL and might be ready before their time on the 60 would expire.