We consider the endogenous selection of strategic contracts in an asymmetric duopoly with substitutable goods. The market includes a biased manager, in terms of the market size the manager expects, in a managerial duopoly with separation between ownership and management. In particular, we focus on how the types of managers selected by owners affect the equilibrium market structures. Furthermore, by considering that each manager's expectation of market size depends on their own type and the type of the rival manager, we examine a situation in which the owner is less certain of the type of manager.