With Biggar in the Wales squad for the upcoming autumn international series, which begins on the first weekend of November, Saints will have to fill the void when he departs.
And that means it will be a big chance for James Grayson to stake his claim for starts.
Grayson was the player introduced at the break at Rodney Parade as Saints secured a 35-21 success in the Challenge Cup clash.
And Boyd said: "James Grayson's going to have to run it for us over the next four or five weeks so it was always a plan to get him on at half-time to see if he could blow some of the rust out.
"It was a tactical substitution (to take Biggar off). He could have stayed on. He took a little knock on the hip, but we were always going to bring Jimmy on to get some time to run it.
"He (Grayson) will be in for the PRC (Premiership Rugby Cup) for the next few weeks and Dan will be away for the autumn series.
"We have a couple of Premiership games Dan will miss as well so we need to bed in an alternative."