He again featured from the bench in last week's Copa triumph over Girona but was replaced after an underwhelming 63 minutes when he returned to the starting line-up for Sunday's 3-0 win over Deportivo Alaves.
With teenage winger Vinicius Junior stringing together some strong performances – including a goal against Alaves – Mijatovic feels Bale could find himself out of favour under Santiago Solari, particularly if he fails to impress against Barca at Camp Nou in the semi-final first leg.
"Somebody has to defend and Lucas [Vazquez] knows how to track back and help. On the left wing, Vinicius is playing well and Bale has his difficulties," Mijatovic told Cadena SER .
"If he [Solari] plays Bale, it's the last chance he has. We're all annoyed with his game. Vinicius plays better at the Bernabeu.
"Barcelona have tremendous attacking power. They're capable of scoring three goals in five minutes. They're the leaders [in LaLiga], they're in the Champions League and the Copa.
"It's a two-legged eliminator. We have to go out and make a good result and take it into account regardless of how it ends. If you get a good result, Barca can also win at the Bernabeu."
Lionel Messi has been included in Barca's squad despite being hurt during their 2-2 draw against Valencia on Saturday, but Mijatovic doubts any risks will be taken with the five-time Ballon d'Or winner.
"He's a key player and he knows himself. He'll tell him [Ernesto Valverde] if he can start, and if there's a risk he'll be on the bench. With the experience Messi has, I don't think he's stupid enough to take the risk," he added.
Real Madrid, meanwhile, find themselves in third, eight points behind the Catalans, with Solari's men currently on a run of four wins in a row in the league.