He didn't hide his thumb injury.
http://fullcount.weei.com/sports/boston/baseball/red-sox/2009/12/21/lowell-clarifies-thumb-situation/
“I was in constant and open communication about my thumb situation from the moment it happened to every week after,” he said, pointing to a foul ball against Cleveland starter Jeremy Sowers as the cause for what is now diagnosed as a strain of the radial collateral ligament in his right thumb. Lowell said that he was asked by the Red Sox to take an MRI on Dec. 7 after being put in a splint at the suggestion of Red Sox trainer Mike Reinold, who had visited Lowell in Miami on Nov. 25.
“I’ve had very open and honest communication with the training and medical staff,” Lowell added. “The day this thing happened it was X-rayed and I told them about the pain, which was Oct. 2. That’s why I didn’t play the other two games [in the regular season]. We taped it up in a way that we thought would help me in the postseason. They thought it might be a strain or a bone bruise. I don’t want to lay blame on anybody because in ‘07 my other thumb got hurt and they thought it was a strain as well and it ended up being that way and everything settled down nice and easy.
“I was in constant contact with the training staff and I told them at the two-week [mark] I didn’t feel a difference with my thumb and at the four-week mark it was the same thing and then at the seven-week mark Mike Reinold flew down to Miami to check it out and that’s when they decided to put me in the splint and keep it immobilized for a month. That’s when the thinking was that it might be something more than it was originally thought to be. The first day of the winter meetings was when I was ordered to get an MRI.