Despite the Blues' desperate need for a win to boost their Champions League qualification hopes, the manager benched star players Nicolas Jackson and Cole Palmer, and they returned home with just a point.
This decision did not sit well with the former England international, who claimed he was flabbergasted by Maresca's tactical choice against the Bees.
"I couldn't believe the team Enzo Maresca put out at Brentford. When that team came out, I was flabbergasted. It was pathetic. Chelsea have won the Champions League, " Merson wrote in his column for Sky Sports.
"This is the Europa Conference League - and he's resting players for it? Getting into the Champions League has to be the be all and end all. They have to be in it. If they come sixth in the Premier League and win the Europa Conference League, it would be a shocker. A shocker.
"This is Legia Warsaw, no disrespect to them but they won't beat Chelsea over two legs. You have to beat Brentford. If they had won that game, then beat Ipswich at the weekend, they would have Champions League football in their hands with six games to go.
"And then look at what happened when those subs like Cole Palmer and Nicolas Jackson came on. They dominated Brentford. But now, they are right up against it. Chelsea might have to finish ahead of Manchester City to qualify for the Champions League, in an era when every team that finished ahead of City won the Premier League."