With Joe Gomez out for six weeks with a leg fracture, Trent Alexander-Arnold was expected to replace him.
But Klopp has opted to play Milner there against the in-form Ryan Fraser.
And the Liverpool manager is comfortable with playing the 32-year-old in defence.
Klopp explained: “It’s no problem.
“I thought it could be a problem, but obviously he would have even played goalkeeper in game number 500!
“We need to flexible, all of us, in this period – different positions, different jobs to do in the games.
“But it’s only slightly different and Millie knows it from his time at left-back.
“We are a very offensive-orientated team, which means he is very often in a comfortable area.
“With his experience, and especially against Bournemouth with the counter-attack, you need all the defensive skills as well.
“Fraser, Brooks, Stanislas or whoever are all nice and speedy, King as well.
“It’s a job to do, but it’s nice to be part of the 500th Premier League game of Millie.”
Milner will be making his 500th Premier League appearance, just the 13th different player to reach the half-century.
At 32 years & 338 days old, he’ll be the second youngest player to hit the landmark.