The German made the surprise choice of starting centre-back Victor Lindelof at right-back at Wanda Metropolitano.
Joao Felix opened the scoring on Lindelof's side after just seven minutes.
Lindelof was eventually brought off in the second-half and United went on to win 1-1.
"To start with, we could do with him (Victor Lindelof) in aerial duels, set-pieces, free-kicks for and against us," Rangnick said.
"All three centre-halves were in good shape, but after seven minutes we were 1-0 down the plan was in the dustbin.
"I knew this was not his best position, when we had Aaron Wan-Bissaka on the ball and Alex Telles, we had more possession and more assistance from the wings. In the end, due to the early goal it was difficult.
"In the second half we showed in quite a few moments how we have to play. It will be different at home in front of 75,000 fans and more conviction than we did in the first half."