The Gunners are in third place in the Premier League table and five points behind leaders Leicester City.
If they lose and Leicester win, then it’s goodnight for the title race
Arsene Wenger’s side suffered a 3-2 defeat by a weakened Manchester United side at Old Trafford on Sunday afternoon.
Arsenal are bidding to win their first Premier League title since 2004 but also trail second-placed Tottenham Hotspur by three points.
Former Gunners midfielder Merson described Wednesday night’s game at the Emirates Stadium as must-win for the north London side.
“I agree with Graeme Souness that Arsenal were a joke on Sunday, but I believe they will beat Swansea at home,” Merson told Sky Sports.
“They simply have to win this because if they lose and Leicester win, then it’s goodnight for the title race.
“It would be an eight-point gap to Leicester and they’re also chasing Tottenham who have a much better goal difference.”
Arsenal are winless in their last three games in all competitions after a miserable recent run.
The Gunners played out a stalemate with Championship side Hull City in the FA Cup fifth round.
Arsenal lost 2-0 to Barcelona in the first leg of their Champions League last-16 clash last Tuesday.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news