Smalling spent 10 years with United in which he made over 300 appearances for the club.
After successfully rebooting his career in Italy, the Englishman will get the chance to face his former side in the Europa League semi-final.
"Yeah, 100 per cent [facing United brings extra motivation]," he said at a press conference appearance on Wednesday.
"It's a special game, it's a semi-final and added on top playing against United.
"Obviously being in Rome for nearly the last two years, I think, I know the importance of bringing home a trophy because it's been far too long.
"There's so many ingredients for this so I don't think there will be a bigger game to be a part of.
"I'm just thankful I'm now fit and I'm able to contribute because the whole squad needs every player."
On United's progression under Solskjaer, Smalling added: "This season they're obviously having a very good season.
"When he first come in I was injured, but then as soon as I got fit he played me and we had a good relationship.
"Then obviously we had very honest conversations in the summer but, yeah, now we'll meet on the pitch and I'm really looking forward to it."