Eden Hazard's penalty had given the Blues a potentially vital away goal, but Javier Pastore's brilliant stoppage-time solo goal, added to earlier strikes from Ezequiel Lavezzi and a David Luiz own goal, saw PSG take a two-goal advantage into next week's second leg at Stamford Bridge.
The two sides will line up again on April 8 with PSG very much in control of the tie, but Motta still reckons Jose Mourinho's men are favourites.
He told L'Equipe: "This is a good result, but it is still nothing done, there is another 90 minutes.
"They remain the favourites, they have such an experience in European Cups, such players. They will be even better on hostile terrain next week.
"The last 30 minutes of the first period (in Paris), they were huge. There were so many key things that happened in the game - the penalty that wasn't given to (Edinson) Cavani, (Zlatan) Ibrahimovic's injury - but we showed we knew how to react and recover."
PSG striker Ibrahimovic pulled up with a suspected hamstring injury and is now a major doubt for the return leg.
When asked about the Sweden international's injury, Motta said: "Ibra? It's complicated, we will wait and hope it is not too serious."
Cavani claimed the tie was very much in PSG's favour thanks to Pastore's late score.
"This is a great victory," the striker said.
"But in football, everything is possible, even if our possibilities are obviously large thanks to the goal at a key moment by Pastore."
However, the 27-year-old Uruguay international said he would not feel any extra pressure to score if Ibrahimovic was ruled out of the Stamford Bridge showdown.
Cavani added: "I will have no more responsibility to score, I have it every game."