Bailly departed Spanish football with a growing reputation and was expected to star in England as he approached his peak.
Time is still on his side at 24 years of age, but injuries and inconsistent form have blighted his time in the Premier League.
Marcelino said of those struggles as he prepares for a Champions League reunion with a familiar face on Wednesday: "I was so lucky to have Eric as a footballer, as a player.
"He’s an extraordinary player and an extraordinary person. I’m so sorry for his injuries, because I know his performances are conditioned by this.
"I know he is having a hard time and suffering when trying to have a stable rhythm in the team.
"I wish nothing but the best for him and I feel his youth will help him through these hard times and he will be very impressive for United.
"I think he’s playing really well for United when he plays, but because of his injuries he’s not getting the luck he deserves."
Bailly has been restricted to just eight appearances for United across all competitions this season, but was welcomed back into a starting berth for a recent 2-2 draw with Arsenal.
He will be hoping to figure more prominently over the remainder of the campaign, having only taken in 18 outings during the 2017-18 campaign.
Jose Mourinho could do with him finding form and fitness, with the Red Devils’ failings at the back well documented.
Only five sides in the Premier League have been breached more times than United this term, with their record currently the joint-worst in the top half of the table.