The Belgium international became the club’s second-most expensive signing of all time at the weekend after Manchester United agreed a £75m deal with Everton to sign the powerful centre-forward.
Lukaku was initially expected to return to Premier League champions Chelsea this summer but the Red Devils managed to capitalise on their rivals’s failure to get a deal over the line.
The 24-year-old scored 20 times in 36 games last season to finish as the Premier League’s second-highest goal-scorer behind Tottenham Hotspur rival Harry Kane.
The Belgium star moved to Everton from Chelsea in a £27m deal after failing to establish himself under Jose Mourinho in 2014.
Now Lukaku will be tasked with replacing Zlatan Ibrahimovic’s goals in the Manchester United team next term.
Speaking to ESPN FC about whether Lukaku will be a success at Old Trafford, former Arsenal striker Mariner said:
“Playing for Everton with a No9 shirt in recent seasons, there isn’t much pressure.
“At Manchester United, the pressure is incredible. It is every single day. The media will be asking questions about Lukaku. They will be observing him. If he can handle the pressure that is one aspect of being a No9 for Manchester United.
“The biggest aspect is in those tight areas and when teams defends deep, does he have the quick feet to open up a defence? I don’t think he does at the moment.”
Manchester United finished in sixth place in the Premier League table last season.
The Red Devils won the EFL Cup and the Europa League in Mourinho’s first campaign in charge.
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