Sevilla travel to Manchester having drawn 0-0 with Jose Mourinho's men at the Ramon Sanchez Pizjuan on February 21 in the first leg of their Champions League round of 16 tie.
It was seen as a missed opportunity for Sevilla, given they controlled much of the game and saw Luis Muriel denied a goal by a remarkable David de Gea save just before half-time.
United have won all three of their matches since drawing in Seville, including coming from 2-0 down to beat Crystal Palace 3-2 and Saturday's 2-1 triumph over in-form Liverpool.
While Sevilla have lost two of their four league games since, with Valencia leaving Andalusia with a 2-0 victory on Saturday.
And Montella is well aware of the task facing his side as they look to reach the Champions League quarter-finals for the first time.
"I am as worried as I was in the first leg, because United have a strong physical and technical level," Montella told reporters on Monday.
"It is normal [to be worried] – they are top-level players and they have technique, physical prowess and strength.
Montella's side head to Manchester with some injury problems, particularly in defence.
Sebastian Corchia and his right-back deputy Jesus Navas are both out injured, meaning Montella is going to have to shuffle his back, though he is adamant he will not be switching to a back three as a result.