Mier is going to be examined in Houston in the next few days.
"We just don't know [the severity] until he's evaluated and we get a clear picture of how severe it was," Nelson said. "It's a pretty significant hamstring injury."
In other injury news, right-hander Jack Armstrong, whom the Astros selected in the third round out of Vanderbilt in last year's Draft, is still wearing a brace after undergoing Tommy John surgery two weeks ago. Nelson said team doctors are pleased with his progress, but he'll miss all of 2012.