The Red Devils clinched the win courtesy of three first-half goals, helping them up to sixth spot on the Premier League table.
Rangnick made six changes to the starting line-up from Monday’s 1-1 draw with Newcastle including Jadon Sancho and Edinson Cavani coming into the XI.
“It was about fresh legs, fresh energy, fresh bodies, more physicality against this team,” Rangnick told reporters.
“We knew how difficult they would be, how persistent they would play and it was a little bit of a war of attrition, a little bit of a difficult one to play.
“Therefore, it was important to me to have six fresh players on the pitch compared to the game at Newcastle.”
United are sixth on 31 points from 18 games, six behind fourth-placed Arsenal with a game in hand.
The situation has led to speculation about United making moves in January but Rangnick said he was not making any major transfer plans.
“My focus is on the current players. We have enough players in the squad, so there is no lack of number of players,” Rangnick told Amazon Prime Sport.
“Transfers in the winter only make sense if they really help you, if they increase the quality, and so far we haven’t spoken about that. My focus in the last couple of weeks was on the current squad.”