England forward Rashford enjoyed a brilliant spell last season when he scored 19 goals in 23 matches for club and country.
The 22-year-old went off the boil when the season resumed after lockdown, however, scoring three goals in 13 appearances.
Former Chelsea, Aston Villa and Millwall striker Cascarino says Rashford ‘has not looked the same player’ since lockdown and believes his form has been impacted by speculation Manchester United will spend more than £100m to sign Sancho.
Borussia Dortmund are determined to keep hold of Sancho but Ole Gunnar Solskjaer is keen to welcome him to Old Trafford in a stunning summer deal.
Rashford played the entire 90 minutes of Manchester United’s opening Premier League match against Crystal Palace on Saturday but was helpless to prevent the team suffering a 3-1 defeat.
Cascarino told The Times: ‘To a man, Manchester United were woeful against Crystal Palace but the most worrying thing about the performance was Marcus Rashford.
‘Joel Ward, the Palace right back, did not give Rashford a kick and we saw none of his typical dynamism in attack.
‘Rashford is a player who sets the tempo for United. If he is making threatening runs, or taking on defenders, then Ole Gunnar Solskjaer’s team look deadly.
‘If he has an off-day, they can appear flat.
‘In truth, he has not looked the same player since lockdown. Mason Greenwood and Anthony Martial were lauded for their goals as United finished the season in style and there has been much speculation about Jadon Sancho arriving.
‘I cannot help but feel that Rashford’s nose has been put out of joint.’
Manchester United were given an extra week off ahead of the new season but produced a poor performance at home to Palace, losing 3-1 in a shock and disappointing defeat.
Reacting to the loss, Red Devils manager Solskjaer said: ‘You can see we are short and they deserved the points. Today was a performance you don’t see very often from this group.
‘We should look at ourselves in the mirror. Everyone knows we can perform better than this.’