The South African, whose last appearance before a hamstring injury was on August 31, brilliantly converted Kevin Mirallas' cross within 30 seconds of coming off the bench.
His goal proved the decider in a win in which Mirallas had seen his early strike cancelled out by Yannick Sagbo.
Pienaar was not expected to return until next week but the manager changed his mind a couple of days ago.
"Thursday's session he looked really sharp and he finished so clinically I felt he was not ready for 90 minutes but we had to have him around the team," he said.
"First touch and back of the net is quite an impact.
"Steven Pienaar brings something really special to our side, for many reasons - firstly the understanding he has with Leighton Baines and also the way he can find space.
"We knew he couldn't play 90 minutes but the impact he had in the game was terrific.
"It was important for us to have that little bit of magic to get that second goal."
- Sports & Recreation
- Kevin Mirallas
- Steven Pienaar