Former Arsenal attacker Merson seems to rate the 30-year-old France international defender highly.
In fact, the centre-back is one of only three Arsenal players the former England international has included in his best lineup from the current Gunners and Manchester United's squads ahead of Sunday’s meeting between the two sides in the Premier League.
“If (Koscielny) he didn't play for us God knows where we'd be,” wrote Merson on Skysports.com.
Arsenal’s defence has been a problem for some time, but Koscielny remains one of the best centre-backs in the Premier League.
The 30-year-old has been on the books of the Gunners since 2010 and is one of the first names on the teamsheet when he is fit.
Koscielny will not play for Arsenal against United at the Emirates Stadium in North London on Sunday, though.
The Frenchman has a hamstring injury and has been ruled out of the match by Arsenal manager Arsene Wenger.