Lahiri, who is now two-over 215 for three rounds, needs to finish inside tied-41 to be able to qualify for the third leg of the Play-offs.
Earlier, Lahiri missed the cut at the Northern Trust, the first of the four events, which will end with the Tour Championships featuring the Top-30 players.
The Indian was 66th on the FedExCup standings going into the second round and needs to be 70 or better.
On the back nine, he bogeyed 10th and 12th and even a birdie on 18th was small compensation on a wet day at TPC Boston.
Meanwhile, PGA champion Justin Thomas ran off eight birdies on a for a tournament-best 63 and that gave him a share of the lead with Marc Leishman going into a Labor Day Monday finish.
Leishman, who won at Bay Hill this year, also played bogey-free for a 65 on Sunday to join Thomas at 12-under 201.
Thomas had his third round this season at 63 or better -- a list that includes his 59 at the Sony Open and a 63 at the US Open -- to turn a five-shot deficit into a share of the lead and a chance to win for the fifth time this year.
He birdied six of the first 10 holes, the longest at about 12 feet. He played down the 13th fairway on the 12th hole for the third time this week and holed a 50-foot birdie putt, and then added a pair of 25-foot birdie putts.
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