Belles have lost all three of their opening Women's Super League One fixtures this season, after winning promotion last year.
"We know we need to strengthen because we're a very thin squad," said Harris.
"We're a couple of player's short and we need to put pressure on the players that we already have."
He added: "Those players in my view have not quite come to the standard that they've shown in training."
Doncaster made seven signings in the summer, including England striker Natasha Dowie.
However, they are currently bottom of the WSL 1 table with no points and a goal difference of minus 10 following defeats by Chelsea,Birmingham and Manchester City.
But Harris insists that the club are confident that they can "regroup" during the month-long break at the end of May.
"If you look at the run of games we've had, we've not had the easiest of starts," he told BBC Sport.
"The morale is good but when you've lost three games on the trot, it does knock your confidence a bit.
"I don't think it's a lack of belief though, I think it's just that little bit of confidence, that little rub of the green to get you going."
Doncaster Rovers Belles manager Glen Harris says the club need to make signings when the transfer window opens on 3 June.