The Inter Milan defender leaned into Armstrong as he was about to take a shot, which could have made a difference during the 1-0 victory for the Serie A giants.
“Yeah of course I felt contact. If there wasn't I'd have taken a shot as there was no need to go down,” said Armstrong.
“I felt an arm on my shoulder and it put me off balance. It's just the way it's gone.”
Celtic will feel extremely hard done by, especially after defender Virgil van Dijk was shown two questionable yellow cards in the first half.
Celtic battled admirably and showed plenty of fighting spirit, but it wasn’t enough to see them grind out a result and force their way into the next round.
Had the Bhoys been awarded a penalty inside the first 15 minutes, the momentum and general direction the match went in might have been very different.
However, it was always going to be difficult for the team to get the run of the decisions against such an imposing opposition in an intimidating ground for any official to experience.
The important factor for Celtic is how they now react this weekend in the SPL clash against Aberdeen.
There are still several chances for silverware for the club this season, with qualification for the Champions League also more than likely. The side will move on and will still have a good season.