Steven Gerrard's Glasgow Rangers were involved in a dramatic Europa League clash with Spartak Moscow last night.
The Gers were 3-2 up by half-time, with both teams scoring own goals.
The ex-Aberdeen ace said: "The defensive side of this game has been poor.
"Flanagan's been suspect anytime Spartak players come near him."
Just playing in the Europa League is a big step up for Flanagan, who was on loan at Bolton Wanderers in the Championship last season.
He featured last night at left-back, filling in for injured Croatia international Borna Barisic.
Rangers led 3-2 at half-time but lost the match 4-3 after conceding twice in two minutes in the second half.
Flanagan was substituted for Andy Halliday in the second half.
He was not the only player to struggle, with Nikola Katic poor on his return and Connor Goldson scoring an own goal.