The ex-Norway defender believes the Londoners need to ditch their regular 4-3-3 system and flood the midfield with players in order to nullify United's strength in that area of the pitch.
"I would go with Didier Drogba up front in a 4-4-1-1 formation, with Nicolas Anelka just behind him," Chelsea's 1995 player of the year Johnsen told Reuters in a telephone interview from his home in Lillestrom.
"I don't think you can play 4-3-3 at Old Trafford... and I would put Torres on the bench."
Second-placed Chelsea trail leaders United by three points with three matches left this season. Both teams also have identical goal differences of plus 38.
Johnsen, a tough as teak defender who also had a spell with Bayern Munich during his playing career, said Chelsea must not repeat the mistakes made by Schalke 04 in the German team's Champions League semi-final defeat by United.
"What Schalke did wrong, especially in the first leg at home, is they left a big hole in central midfield," said the tall Norwegian, who made 183 appearances for the London club between 1989-97.
"You need to be really tight in midfield against United and Chelsea need their two wingers tucked in to lend a hand there.
"When Chelsea play 4-3-3 their wingers are too high up the field. If they don't have them helping out in midfield at Old Trafford they will be in deep trouble," added Johnsen, now head of youth development at Norwegian top-flight club Lillestrom.
"Chelsea will need to be tight in defence and hit United on the counter attack."
