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The visitors, who were 12th in the league prior to kickoff, had the better of the early chances as Vincent Aboubakar dragged wide from 12 yards and Yann Jouffre tested Salvatore Sirigu with a long-distance drive.
The French giants were drawn against the Blues in the Champions League quarter-finals on Friday, and whether Silva will be fit or not, they have the talent at the other end to trouble Jose Mourinho's side.
Edinson Cavani twice came close with headers as the visitors pushed for an opening goal at the Stade du Moustoir, while Zlatan Ibrahimovic's thumping shot was kept out by Lorient keeper Fabien Audard.
PSG's pressure paid dividends when Motta poked the ball home on the goalline in the 41st minute to net his fifth goal of the season, and they continued to pepper the Lorient net after the break.
Ibrahimovic collected a Marco Verrati pass and shot towards the bottom corner, only to see Audard deny him again, while Marquinhos headed wide from Motta's cross on 53 minutes.
Their failure to increase their advantage nearly cost PSG two points when Aboubakar nodded narrowly over the crossbar in the final five minutes, as Laurent Blanc's side hung on to claim victory.