In the Mitchell Report it says that Theo had questions about Gagne's past steroid use and was told he could break down.

When the Boston Red Sox were considering acquiring Gagné, a Red Sox official made specific inquiries about Gagné’s possible use of steroids. In a November 1, 2006 email to a Red Sox scout, general manager Theo Epstein asked, “Have you done any digging on Gagne? I know the Dodgers think he was a steroid guy. Maybe so. What do you hear on his medical?”

The scout, Mark Delpiano, responded,

Some digging on Gagne and steroids IS the issue. Has had a checkered medical past throughout career including minor leagues. Lacks the poise and commitment to stay healthy, maintain body and re invent self. What made him a tenacious closer was the max effort plus stuff . . . Mentality without the plus weapons and without steroid help probably creates a large risk in bounce back durability and ability to throw average while allowing the changeup to play as it once did . . . Personally, durability (or lack of) will follow Gagne . . .