Bale has not played for Real Madrid in six weeks after suffering a calf injury.
Isco has deputised in Bale's absence and the Spain international has been excellent, playing a key role as Madrid won La Liga.
But Bale has returned to training just in time for the final in his homeland.
Madrid boss Zinedine Zidane's choice between Bale and Isco has dominated the headlines in Spain and Balague says Bale will be on the bench after "key players" made clear their preference for a four-man midfield against Juventus.
Gareth Bale in Real Madrid training ahead of Champions League final Wed, May 31, 2017
Express Sport brings you the best pictures from Real Madrid's latest training session ahead of the Champions League final against Juventus
Play slideshow Getty Images 1 of 10
Real Madrid were hard at work in training ahead of the Champions League final
He tweeted: "Gareth Bale will be on the bench. Isco will start.
"Four man midfield for Real Madrid was preferred by key members of squad."
In his press conference yesterday, Zidane refused to give away who will start.
Gareth Bale will be on the bench. Isco will start. Four man midfield for Real Madrid was preferred by key members of squad pic.twitter.com/zauZDHWjt8
— Guillem Balague (@GuillemBalague) June 3, 2017
Gareth Bale looks set to start the final on the bench
Get Quotes on Home Insurance
He said: "I'm not going to tell you who's going to play tomorrow.
"Isco and Bale are two players who can also play together; in the long run, anything can happen.
"When I look at my squad, what's good is that they're all ready and willing to play. It's not only mentally, of course, they have to be ready physically too.
"That's what I'm interested in right now, everyone in good shape and at a high level."