Caleb Thielbar (hamstring) threw a successful bullpen session on Friday.

The 39-year-old southpaw is working his way back from a left hamstring strain. He’ll repeat the process again on Sunday and then progress to a full bullpen early next week. If all goes well, he could be cleared to begin a minor league rehab assignment the following week. Barring any setbacks along the way, Thielbar should be able to return to the Cubs’ bullpen before the month of May is through.