The Blues travel to Old Trafford on Saturday, having won just two of their first five league matches of the season.
Big-spending Chelsea were beaten by defending champions Liverpool in September and then dropped points at newly-promoted West Brom.
A 4-0 victory over Crystal Palace followed but Chelsea’s defensive problems were exposed again in a disappointing 3-3 draw with Southampton last weekend.
Manchester United have also struggled to find consistency this season but face Chelsea on the back of Tuesday’s impressive Champions League victory over PSG.
Arsenal legend Merson believes the pressure is on Lampard ahead of Saturday’s clash at Old Trafford and says the Blues boss will not get ‘loads of time’ this season due to the summer spending spree at Stamford Bridge.
He told Starsport: ‘I think this is a big match for Frank now against Manchester United.
‘A few days ago you were thinking this was a big week for Ole Gunnar Solskjaer.
‘You were wondering whether he’d survive it. But three late goals at Newcastle and a great performance against PSG and everything has changed.
‘Now, all of a sudden the pressure is on Lampard. His side drew 3-3 with Southampton and then couldn’t score against Sevilla.
‘I don’t think he’s going to get loads of time this season because of all the money they’ve spent, and he needs to start getting it right.’
Former Arsenal and England star Merson also reacted to Petr Cech’s shock inclusion in Chelsea’s Premier League squad for the 2020-21 season.
He added: ‘Petr Cech’s chances of actually playing again for Chelsea are absolutely minimal even though he’s been named in their Premier League squad.
‘But if I’m an 18-year-old goalkeeper trying to make my way at the club, I’d be wondering what chance I’ve got if they can bring Cech out of retirement.
‘Chelsea have done really well bringing through their young players under Frank Lampard the last couple of years. They should keep doing it.’