The Red Devils defender was a regular at centre-half under Louis van Gaal during the Dutchman’s two season reign.
Indeed, Smalling became one of Manchester United’s key players and was even handed the captain’s armband to underline his importance.
However, Van Gaal’s successor Mourinho has been reluctant to start with Smalling following the additions of Eric Bailly and Victor Lindelof.
Mourinho hasn’t been afraid to criticise the England international during his spell in charge of the 20-time English champions.
Smalling has been linked with a potential exit at the end of the current campaign given his well-documented struggles.
Former Manchester United defender Parker believes Smalling’s slump in form can be accredited to Mourinho’s treatment of the Englishman.
“Chris Smalling had a very good season under LVG prior to Jose and won the captain’s armband,” said Parker, speaking to 888sport.
“As much as you suggest he isn’t trusted this is a new-fangled word that’s brought up now – trust. Everybody has trust in people or doesn’t and I find it a strange, condescending word to use in football.
“Managers say it about their players but no, they should make them better and make them believe they can do anything. There is a reason why Chris Smalling has been a Manchester United player for so long and there is a reason why he’s suddenly making so many mistakes.
“Ask yourself what are the reasons why Smalling is now maybe jittery? Is it because someone doesn’t have faith in him and so he doesn’t feel comfortable, thinking that every game could be his last?
“Prior to Mourinho he was a regular and as the old saying goes you don’t become a bad player overnight. Does he feel he has enough trust from those around him and the people upstairs? They are the ones who need to show belief in him and let him know why he’s a Manchester United player”
Smalling netted Manchester United’s winner in a 3-2 victory over Manchester City in Saturday’s derby.
Sir Alex Ferguson signed Smalling in a £10m deal from Fulham in the 2010 summer transfer window.
The Manchester United defender has netted 15 times in 281 games in all competitions over past eight seasons.
The Red Devils are in second place in the Premier League table but a mere two points ahead of Manchester City.
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