The England international fell down the pecking order after the signing of Harry Maguire from Leicester City, prompting the two-time Premier League winner to sign for the Italian giants.
Speaking to Sam Wallace of the Telegraph (h/t the Manchester Evening News' Daniel Murphy), Smalling said it would have been easier to stay at Old Trafford, but he is enthused by the challenge of playing in Serie A.
"He [Solskjaer] said I would get games.
"I am used to playing when I am fit. Although that might have been slightly different this season, I would have had the cup games and then in the league, depending on injuries.
"I had spoken to the manager before Harry (Maguire) came and I knew I was behind him and Vic (Lindelof) but in previous seasons I had started behind (two first-choice centre-backs) and after four or five games I had worked myself into the team.
"I love a challenge and when a new centre-back comes in it makes you raise your game. You have to show more, you have to be more consistent.
"If I had stayed I would have taken on that challenge and I would have been very confident that I would have played the number of games that I was worthy of. Equally I have a challenge here at Roma."
The opportunity for a temporary switch to Rome came in the final days of the Italian transfer window, and the Stadio Olimpico team paid €3 million (£2.7 million) on August 30 to sign Smalling for 12 months, according to BBC Sport.
The London-born player failed to impress after the sacking of Jose Mourinho last December, and with Phil Jones and Marcos Rojo remaining at Old Trafford, it was unlikely he would have many chances to feature this term.
Simon Stone of BBC Sport tweeted Wednesday United supporters now consider Victor Lindelof as the natural choice to partner Maguire in defence:
A strong campaign with Roma could see Smalling resurrect his United career next season, but it is possible he has played his last game for the Red Devils.
Solskjaer is in the process of replacing senior members of the squad with burgeoning talent. Romelu Lukaku and Alexis Sanchez departed for Inter Milan in the summer, allowing increased game time for new signing Daniel James and academy starlet Mason Greenwood.
At 29, Smalling has plenty of years left in his legs, and it was a brave choice to move abroad to prove his quality.
Jones and Rojo are likely to experience similar career choices in future transfer windows, and they could follow Smalling out of the Old Trafford exit.
Maguire has performed well in his opening games, but the jury is out on Lindelof. The Swede signed a new deal to stay in Manchester until at least 2024, but the 25-year-old must prove his consistency over the next 12 months.
Axel Tuanzebe is waiting in the wings, and the 21-year-old has the talent and the temperament to become a top-class defender for the Red Devils.