O'Neill was removed in the ninth inning of Sunday's game against the Red Sox due to left hamstring tightness, Jim Hayes of Bally Sports Midwest reports.
O'Neill, who went 3-for-4 with a run scored and a stolen base on the afternoon, suffered the injury while he was running to second base on his sixth double of the season. If the hamstring issue is significant enough to force O'Neill to the 10-day injured list, Juan Yepez would likely be the top candidate to replace him in St. Louis' everyday lineup.