Cosgrove has only just returned to Wales to replace Herschelle Gibbs
Lancashire's final hopes of silverware this season all but ended in Cardiff as bottom of the table Glamorgan pulled off a shock Pro40 Division Two win.
Needing to win to set up a probable final-day Old Trafford title decider against Warwickshire, Lancashire succumbed to a painful 27-run defeat.
Mark Cosgrove starred in Glamorgan's innings, hitting two sixes in his 73 off 65 balls to help run up 219.
But, despite Steven Croft's 50, Lancashire stumbled to 192 all out.
Only a freak set of results, chiefly involving a complete wash-out at Canterbury when Kent play Warwickshire on Wednesday night, can now earn Lancashire the title.
Even then, they would need current leaders Middlesex to lose heavily at home to Northants on Saturday. And Lancashire would then have to inflict a crushing defeat at home to Warwickshire on the final day.
But, effectively, their hopes were ended by Glamorgan's first win - and, to a large extent, it was self-inflicted.
After a flying start from openers Paul Horton and V V S Laxman, they went in quick succession, followed by Francois du Plessis on his final appearance, former Lancashire second teamer Chris Ashling taking two of the wickets on his debut.
A 52-run fifth-wicket stand between Croft and Mark Chilton then looked like turning it round. But Croft was out leg before for 50, reverse sweeping. And, from 129-3, Lancashire subsided to 192, with six of their batsmen either run out or stumped.
Ashling claimed two run outs on his one-day debut, while stand-in skipper Mark Wallace produced three stumpings.
After Cosgrove had launched the Glamorgan innings in typically attacking style, the home side had earlier collapsed themselves, tumbling from 135-2 to 154-7 in the space of just five overs. But an eighth-wicket recovery between Dean Cosker and James Harris navigated Glamorgan to what proved a winning score.
Like Glamorgan, who were without the likes of skipper Jamie Dalrymple, Robert Croft and Michael Powell, Lancashire made an encouraging start.
But after Horton was trapped in front by David Harrison, Lancashire lost wickets at very regular intervals and then three for nine runs as the later middle order disintegrated.
The key wicket of Laxman was claimed by Ashling, who then ran out Chilton and skipper Glen Chapple in the space of three overs, sandwiched by Gareth Cross being stumped by three yards as panic set in in the Lancashire dressing room.
Lancashire coach Peter Moores told BBC Radio Manchester:
"If we'd won, it would have set up a big game a week on Sunday.
"But credit to Glamorgan. 220 was gettable, as we pulled them back after Mark Cosgrove had played very well up front.
"Francois du Plessis was unlucky as the ball came back off the keeper's gloves.
"But we caused our own demise in having three run-outs and three stumpings."
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