The Astros activated Allen (hamstring) from the 10-day injured list Monday, Chandler Rome of The Athletic reports.
Allen has missed the past three weeks with a left hamstring strain, but he's been cleared to rejoin the active roster after playing two rehab games with Triple-A Sugar Land. The 27-year-old is expected to operate as a reserve infielder, though he'll start at shortstop and bat eighth in Monday's series opener versus the Twins while Jeremy Pena takes a seat due to leg discomfort, per Chandler Rome of The Athletic.