Last season’s Conference League winners should have gone behind inside two minutes, when Mikael Ishak - Lech’s all-time top scorer in European competitions - scuffed a shot wide from Patrik Wålemark’s cross with the goal gaping.
Palace responded with a period of heavy pressure, but they were gifted the lead after 10 minutes when Terry Yegbe inexplicably sent a throw-in into the path of Yéremy Pino, who slotted home the Eagles’ maiden UEL goal to end his 27-game goal drought.
Lech’s supporters were in full voice all night and they should have been celebrating an equaliser in the 21st minute, when Ishak lifted a half-volley over the crossbar after being sent through one-on-one.
Instead, the visitors found themselves 2-0 down six minutes later when Chris Richards scored his first goal in European competitions with a header from Adam Wharton’s corner.
It was the sixth time Palace had scored 2+ goals in their past seven European home matches, and they had another six minutes later after Pino was tripped in the box by Radosław Murawski, with Jørgen Strand Larsen firing the resulting penalty into the top corner for his fourth goal in three games.
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Yegbe then inadvertently cleared the ball into his own net from 30 yards, but was saved by an earlier offside against Strand Larsen.
The two teams continued to attack at every opportunity in the second half, with Palace looking to increase their lead and Lech aiming to give themselves a lifeline by scoring in a 16th successive European away match.
Pino was denied his second goal of the night by a brilliant sliding block from Michał Gurgul in the 66th minute, but the most notable moment of the second period came when Palace striker Ismaïla Sarr - last season’s top scorer in the Conference League - made his first appearance of the campaign as a late substitute after recovering from a groin injury.
Eddie Nketiah also entered the fray and dragged a good chance past the far post before Walter Benítez made a sharp low save from Gurgul, but Lech’s night went from bad to worse when Murawski was sent off late on for a second booking.
Nketiah sealed victory with a finish into the bottom corner one minute from time, as Palace claimed only a second win in their last six outings at Selhurst Park (D1, L3).
The Eagles have gone seven home games without losing in Europe (W5, D2), while Lech’s six-match unbeaten away run on the continent is over after suffering consecutive defeats for the first time since March.
Flashscore Man of the Match: Yéremy Pino (Crystal Palace)