The 23-year-old was Sons boss Ian Murray's first signing at the Bet Butler Stadium but has struggled to break into the starting line-up in recent weeks.
He will spend the next month in the Irn-Bru Third Division with the Bully Wee and Murray has wished him well.
He said: "It has been hard for Marc because of the lack of games he has had a chance to play in and he came to me and asked if he could go out on loan to get sharp.
"It is better having someone who wants to play and improve himself than not, so fair to play to him.
"I have no problem with him going out on loan and he will be back before the end of the season.
"He will go there for 28 days and we will see how he gets on, hopefully he does well."
McCusker could make his third Bully Wee bow when Jim Duffy's side face Elgin on Saturday, leaving Dumbarton to battle their way up the First Division standings.
The Sons face crisis club Dunfermline this weekend and while Murray has smpathy for the Pars' off-field plight, he insists his side will show no mercy at East End Park.
He said: "It will be a strange game on Saturday and a lot of it will depend on the size of the crowd, it wouldn't surprise me if they get 5000 or 6000.
"It will be a difficult game for us, in hard circumstances for Dunfermline, but it is one we have to go and win. It sounds brutal but it doesn't have anything to do with us.
"We do feel sorry for Dunfermline and we wish them well but they wouldn't lose any sleep if we were relegated so we need to look after our own business.
"We can feel sympathy for them but we need to be professional on the pitch and get the points for ourselves."