Mourinho managed Smalling for two-and-a-half seasons during his tenure as Manchester United manager.
The pair’s relationship took a hit in 2016 when Mourinho publicly criticised Smalling for failing to play through the pain barrier.
They will have the chance to work again in the Italian capital after Mourinho was appointed Roma’s new manager over the summer.
Despite their little fracas in the past, Smalling maintains he is excited by the former Chelsea and Real Madrid manager’s arrival although he was surprised by his appointment in the first place.
"Ultimately, if the manager wasn't speaking about you and didn't want you to play and push, that is when you need to worry because he has stopped caring about you, he doesn't want you involved and he doesn't want you to play," Smalling told BBC Sport.
"He is a born winner. He likes to push everyone to the limits. That is what he has done throughout his career and it has reaped results.
"It was quite a surprise to a lot of us, but there was initial excitement from everyone. I knew the media would build up something about our relationship in the past. But from a personal point of view, it is a good thing because I have played under him.
"We have won trophies together. He made me captain in one of those finals - the 2017 EFL Cup final - as well, so having the chance to play under someone who you know is good, has been so successful and is so determined to win trophies at all costs is a positive.
"I know how big a deal it would be to bring a trophy of some sort to the club. Jose's history shows the club has picked the perfect man to do that."
