Indeed, the 23-year-old winger, who joined the Bhoys in January after moving from United along with Gary Mackay-Steven, says the size of Celtic as a club as well as the amount of support is the biggest difference between the two sides.
That has obviously impacted Armstrong since his move to Ronny Deila's Hoops, although he says it hasn't been too daunting for either him or Mackay-Steven.
The pair have been in fine form since joining Celtic and he admits time seems to have flown since January.
Armstrong is also delighted by the praise the former Dundee United duo have received since linking up with the Scottish champions, explaining that it was always his aim simply to settle in and make an impact as soon as possible.
SA on biggest difference on playing for Celtic: “The size of the club, playing at Celtic Park every other week, the amount of support." (MH)— Celtic Football Club (@celticfc) April 10, 2015
SA: "It’s gone very quickly, seems to have flown by. But myself & @Gary__11 have made a good start so hopefully it can continue." (MH)— Celtic Football Club (@celticfc) April 10, 2015
SA: “Nice to hear people have said we have fitted in well and not looked out of place, that was the main aim coming here." (MH)— Celtic Football Club (@celticfc) April 10, 2015
Celtic face Inverness Caledonian Thistle on Saturday afternoon during the early kick-off, as they look to extend or at least maintain their comfortable lead at the top of the Scottish Premiership.