johnstons performance clause!?

After Saturdays mishap, when AJ never made a sub till the 85th minute (again!).We were on top but losing steam badly, should've changed it about 65 mins as u do, pasquali, gardyne, eremenko dead in the water, needing fresh legs. But no
waited till after partick scored then took off mckenzie who could run all day!!

I got to thinking surely if it was good enough for shiels then a rookie premiership manager must also have a performance clause, i.e 11th or 12th your contract is discontinued!>:)