Posh suffered a 1-0 defeat at the hands of Barnsley at the Weston Homes Stadium on Tuesday night.
Peterborough United were undone by a deflected Caylan Vickers effort just before the break as Barnsley continued their fine start to the campaign with a narrow 1-0 victory at the Weston Homes Stadium.
Darren Ferguson's men hit the bar and created a number of good opportunities in front of goal, but the visitors also struck the woodwork late on and forced Vincente Reyes into a number of saves.
Carl Johnston had the first effort of the game, but it flew over the crossbar while Kelly did the same at the other end from the edge of the box.
Kyrell Lisbie felt he should have had a penalty kick after going down under a challenge from Lisbie on 22 minutes, but the referee gave a free-kick to the Tykes.
David McGoldrick fired over from the edge of the box before Lolos was expertly denied by Cooper after a quick throw-in.
On 43 minutes, Barnsley opened the scoring as VICKERS cut inside onto his left-foot and his curling shot was deflected past Reyes by the head of Okagbue.
Khela cleared off the line on the stroke of half-time from a low McGoldrick strike while Phillips came close minutes into the second half.
On 48 minutes, Posh were a whisker away from levelling as Khela struck the crossbar from close-range while Ihionvien also came close at the near post.
Reyes kept out a low drive from Connell on 57 minutes with substitute Abrahma Odoh firing over from 25-yards. On 74 minutes, McGoldrick struck the post and then seconds later, he fired wide.
Reyes made a fine stop in stoppage time to keep out Phillips one on one before substitute Gustav Lindgren failed to convert a low cross from James Dornelly.
Posh: Reyes, Johnston, Collins (sub Lindgren 75min), Lolos (sub O’Brien-Brady 65min), Khela, Ihionvien, Nevett, Lisbie (sub Odoh 65min), Hayes (sub Frith 59min), Okagbue, Dornelly. Unused subs: Smith, Mills, Mendonca.
Barnsley: Cooper, Shepherd, Vickers (sub Cleary 89min), Phillips, McGoldrick, Ogbeta (sub Bland 74min), Kelly (sub Russell 83min), Watson, Earl, Keillor-Dunn, Connell. Unused subs: Flavell, Farrugia, Barratt, Farrell.
Half-time: Peterborough United 0-1 Barnsley
Full-time: Peterborough United 0-1 Barnsley
Attendance: 7,148 (994 Barnsley fans)