Are they though?
Cabango, Key, Tymon, Fulton and Cullen all date back to the Duff era (three played under Martin), all have played in sides that have mostly struggled, and they've all had several spells of poor personal form.
Then you have players like Eom, Vip and Ronald who have been hot and cold ever since coming here during the time Williams was here.
So you can reasonably say that Sheehan is not the problem with them. They have always been a bit iffy.
The new players have only been here a couple of months, it's too early to judge how good they are or not regardless of who the coach is.
Bottom line is, there's little evidence to support the statement you're making.