The Catalans beat third tier Cartagena 4-1 in a first leg Copa del Rey tie on Friday, but they had to come from behind to record only their first win in the last three matches.
Having been defeated by Ajax and Athletic Bilbao in their previous Champions League and La Liga matches respectively, and with the team still searching for fluidity without the injured Lionel Messi, Armstrong feels his countryman Neil Lennon could hardly have picked a better moment to take his side there.
"Tata Martino has changed this Barcelona side," said Armstrong. "They play a lot more long balls, right-to-left diagonals and left-to-right diagonals, which is something you can use but not as frequently as they have been using it.
"They are not playing through the midfield as much because Messi is out. Cesc Fabregas is probably the best equipped to play the role that Messi plays because he creates stuff from nothing.
"But Xavi and Iniesta haven't been so prolific without Messi in the last few games, so I think the team in general is in a bit of a confidence crisis. They've not played well recently. Bilbao deserved to beat them. The defence is still vulnerable, with Javier Mascherano still playing centre-half even though he is a midfield player, and they can be caught out at set-pieces. They are still not the best in the air when it comes to corner kicks and free-kicks, but Neil and Celtic will know that."
These travails, however, need to be put into context. Barcelona still lead La Liga, albeit on goal difference, from Diego Simeone's Atletico Madrid, have already qualified for the last 16 and require one point to do so as group winners. Exactly how strong a side they field on the night remains to be seen, but Armstrong sees no reason for any lack of motivation.
"They won't want to lose too many games back to back because it doesn't do their confidence any good," said Armstrong. "But it is not a bad time for Celtic to go to the Camp Nou. A draw would be a fantastic result for Celtic, and I can see them getting it. Of course Barcelona will still fancy themselves to win the league, but Real Madrid are getting better and Atletico Madrid are the ones they really have to be careful of," said Armstrong.