The Saints are seeking a first major honour since 1976 in Sunday's Wembley final, but find themselves coming up against Ibrahimovic and a United side in fine form.
However, while acknowledging the forward's talent, Forster says Claude Puel's side will prepare as if it were any other game.
"We've all seen a lot of Zlatan since he's come here and before that," he told Sky Sports. "We all know what a world-class player he is, but [United] have got world-class players in every position.
"You just prepare as if it was a league game against Man United. They've got quality all over the pitch and it's too important to just get focused on one individual."
Forster believes the key to getting a result for Southampton will be striking the right balance between defence and attack.
"But when you come up against that calibre of team – Manchester United are similar with a lot of ball possession and a lot of quality players – if you go expansive, it gives them the opportunity and more space to work.
"Equally, it's hard to go the other way and defend for 90 minutes and try to hit them on the counter. It's the manager's decision what we do and we'll be 100 per cent at it when matchday comes."
Puel's men were beaten 2-0 at Old Trafford in the Premier League in August.