Piper Jaffray analyst Matt O'Brien says that while the departure of Abiomed CFO Mike Tomsicek is a surprise, it is not likely indicative of any negative factors. Management indicated Tomsicek's decision was not a result of any disputes or accounting/financial issues, O'Brien tells investors in a research note after speaking to the company. He notes that Abiomed, importantly, reaffirmed guidance for the year. The analyst recommends buying the stock aggressively on any pullback today and keeps an Overweight rating on the shares with a $160 price target. Abiomed is O'Brien's favorite mid-cap name.