The back-rower suffered a hamstring injury in the club’s first pre-season fixture against Premiership outfit Newcastle Falcons at Kingston Park on Saturday.
Director of rugby Clive Griffiths said: “It would be a massive blow if he misses the start of the season. He is the last person we’d want to lose.”
Hills limped off in the last of the four 20-minute quarters after playing his part, along with James Fitzpatrick, in the try scored by back-rower Max Davies, which cut Newcastle’s lead to 22-12.
The home side added two further tries, one of which was converted, to run out 34-12 winners.
Knights, who gave a a run out to all their fit players including veteran forwards’ coach Glen Kenworthy, were slow out of the blocks and Falcons twice went close in the first quarter.
It was against the run of play when they opened the scoring in the last minute of the first quarter. Mat Clark, who had featured in the move a couple of minutes earlier which saw fellow centre Lloyd Hayes go close, again played a prominent role.
One of the stand-out players during the club’s record-breaking 2015-16 campaign, latched on to a loose ball on the halfway line and raced away before sending full-back Sean Scanlon over for his first try in Doncaster colours.
Dougie Flockhart added the extras to make it 7-0 but Falcons hit back with two tries by hooker Scott Lawson from catch and drives to earn his side a 12-7 lead in what was proving to be a a typical early warm-up games punctuated by handling mistakes and messy set-pieces.
Knights looked to have drawn level but back-rower Glen Young failed to ground the ball properly against his parent club who rode their luck before adding a third try before the break to lead 19-7.
A much-changed Knights edged the third quarter only to fall further behind from a penalty.
“I don’t think the score-line reflected the game with their last try coming at the death when we had a number of trialists and a couple of their players on.” said Griffiths.
“It was our first game for 80 days and it showed at times - whichwas always going to be the case - whereas they had played last week.
He added: “There is obviously a lot of things to work in training this week –particularly in terms of our attack which was a bit narrow – prior to the Canada game.
“What did please me was our defence. Three of their tries, including the one at the end, came from catch and drives.
“They reason they elected to kick to the corners for a lineout (rather than take a tap penalty) was because they struggled to break our defensive line.”