The forward came off at half-time of United's Europa League game against FC Porto.
Many fans thought Rashford may have been injured, but Ten Hag confirmed it was rotation.
“He is okay,” the Dutchman told MUTV, warding off any worries of an injury issue. “But also Alejandro Garnacho, he is, in the season, our attacking threat in this moment, and we brought him from the bench.
“We need fresh players also for Sunday so, yes, it was all in the plan.”
He added to TNT Sports: “We have to rotate. Garnacho, we didn't start him. He had a great game, not only on Sunday, but I think in the whole season. We sub them because on Sunday we have another hard game, a good game, so we need players to be available and be fit and be fresh.
“It's a quick turnaround to Villa. They have a day longer (to recover) and we have an away game, so we want fresh players on the pitch.”