Ibrahimovic struck twice in the first half as PSG defeated Saint-Etienne 2-0 in Paris to maintain their eight-point lead at the top of Ligue 1.
While PSG coach Laurent Blanc was 'a little annoyed' with his team's performance, Galtier said the visitors could not match the capital club due to Ibrahimovic - 'the killer of opposition sides'.
"PSG were very, very strong and wanted to kill the game quickly. We couldn't respond to their intensity and their pressing," Galtier said.
"Once we were behind it was always going to be difficult. Nevertheless, I hoped we would only be one goal down at the break.
"Unfortunately we came up against a very good Paris side who had a phenomenon up front - Ibra, the killer of opposition sides.
"In the second half we played more like we wanted to. We maybe deserved to get a goal back and that would have made for a tense finish but we couldn't take our chances. Paris deserve their victory."
Ibrahimovic gave PSG the lead in the 14th minute, lashing home Blaise Matuidi's pass, before the 32-year-old Swede notched his 40th goal of the season in all competitions, deflecting Edinson Cavani's cross past Saint-Etienne goalkeeper Stephane Ruffier four minutes before half-time.
While Blanc was happy with his side's opening 45 minutes, the PSG boss labelled the hosts 'distinctly average' after half-time.
"It wasn't a great performance. We were on top in the first half but we were distinctly average in the second half," the 48-year-old said.
"I am a little annoyed about that but that's the way it is. Sometimes you can be not at your best.
"We had chances to make it 3-0 that would have killed the match. It was an open game against a Saint-Etienne side who were not afraid to play. Subconsciously we were happy to just see the game out in the second half."
PSG lead Ligue 1 with 70 points from 29 games, eight ahead of second-placed Monaco, while Saint-Etienne suffered their second straight loss to stay fourth on 48.
