The 27-year-old’s error in the 65th minute allowed Panutche Camara to steal in and square to Ryan Hardie – who bagged the only goal of Tuesday night’s game.
Despite the error costing the Blues, his appearance in Devon was a 30th of the season, representing his best return in a campaign for seven years.
However, under Danny Cowley's handling, he has managed his most appearances since amassing 38 for the Robins in their 2014-15 League One campaign.
For Thompson, it caps off an impressive rise from bit-part League One player, to influential first-team regular at Fratton Park.
When he arrived in the summer, his injury history was a cloud that loomed large, as he struggled for minutes.
Indeed, it took him until mid-September to make his first league appearance, now he has established himself as a Pompey regular and, crucially, his body is holding up.
After impressing for the Robins in 2013-14, Norwich swooped for his services in September 2014, before loaning him back to the club.
He then continued his impressive rise in central midfield under Mark Cooper, by making 38 appearances in all competitions – as the Wiltshire side missed out on promotion following play-off final defeat to Preston.
Another loan deal back to his former youth team side was agreed the following year, yet injuries started to take their toll.
In 2015-16 he made 29 appearances, followed by only eight upon his return to Carrow Road the next season.
Disaster struck in late 2016 of that campaign, when an Achilles injury cutting his season short and hampering progress for the Norfolk side.
As a result, he made just 39 appearances in the next three-and-a-half years during loan spells at Shrewsbury and MK Dons before Norwich released him last summer.
Thompson has publicly stated his desire to stay, while Cowley has shared his view – with the club able to trigger a year-long extension in his contract.
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