Wales international McCusker, 30, has impressed in the back row since a return from injury last month.
Eleventh-placed Irish face Northampton on Saturday, looking for a first Premiership away win in almost a year.
"We'll be keen to stay in touch with them as much as possible," McCusker told BBC Radio Berkshire.
Irish sit level on points with fellow strugglers Newcastle ahead of the trip to Franklin's Gardens and will be keen to maintain the momentum of their recent home form.
"We've had some games recently where we've been in it or just behind at half-time," said McCusker.
"But in the second half we've had games where we've just capitulated and been a totally different team.
"If we let Northampton get too far ahead, they'll be buoyed by that."
The former Scarlets captain left the Pro 12 club in September by mutual agreement as he joined Irish on an initial one-year deal.
Capped six times by his country, McCusker admitted the decision was driven by a need to play more games.
"I didn't really know how I'd ever leave," he said. "But circumstances changed and it wasn't the place I had fallen in love with.
"So it made the decision a lot easier. It was probably the right time and felt like the right thing to do."

Australian Border Force (ABF) said its officers would be checking for visa fraud as part of a general crime crackdown involving several agencies.
But the move sparked a social media outcry, with fears of harassment and racial profiling.
Victoria Police then said Operation Fortitude had been cancelled.
A scheduled press conference was also cancelled after about 300 people gathered in the heart of Melbourne to protest against the operation, The Age reported.
The initial statement from the ABF said a "diverse team of transport and enforcement agencies" would be operating in Melbourne on Friday and Saturday night.
"Officers will be positioned at various locations around the CBD speaking with any individual we cross paths with," said regional commander Don Smith.
"You need to be aware of the conditions of your visa; if you commit visa fraud you should know it's only a matter of time before you're caught out."
Word of the plan spread like wildfire on social media, and there were calls for a snap protest outside the Flinders Street train station.
The ABF later issued a "clarifying statement" on its plans, saying: "[We] will not be 'stopping people at random' in Melbourne to 'check people's papers' as reported in media."
"The ABF does not and will not stop people at random in the streets and does not target on the basis of race, religion, or ethnicity."
Police have since cancelled the entire multi-agency operation.
"Victoria Police has made a decision not to go ahead with this weekend's Operation Fortitude," it said.
"We understand there has been a high level of community interest and concern which has been taken into consideration when making this decision."
The Federal Government spent $10m Australian ($7.7m; Â£4.6m) in branding the months-old Border Force with a new hardline image, local media this week reported.
It including new military-style uniforms and signs at airports and detention centres.
London Irish number eight Rob McCusker believes confidence can flourish among his team-mates after their crucial Premiership victory against Worcester.
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Australia's border protection agency has sparked outrage with a snap announcement it would carry out visa checks on the streets of Melbourne.