A series of abusive Twitter remarks were posted on an account on Tuesday, directed to Flower, who was sent off in Saturday's Super League Grand Final.
Flower received a six-month ban for hitting St Helens stand-off Lance Hohaia.
One comment on the single account said Flower's "last days are approaching".
Other Twitter messages said that Flower "deserves to be shot", adding that he  is a "dead man".
Greater Manchester Police (GMP) tweeted: "We've been made aware of abusive social media activity about the Rugby League Grand Final. Please be reassured that we are looking into this."
More than 70,000 fans at Old Trafford and millions of television viewers saw the player hit Hohaia, who lay motionless on the ground just two minutes into the match.
St Helens chairman Eamonn McManus said after the ban: "We must assist [Flower] in any way that we can and accept him back into the sport. There is a person behind the player, and I am reliably informed by all that it is a good one."
Mr McManus also said of the punching incident: "These matters have to be dealt with by the appropriate body within the game and it has so been."
A GMP spokesman said any decision to take action against Flower would depend on conversations it had with St Helens.
Death threats made against Wigan rugby league player Ben Flower, who punched an opponent as he lay on the ground, are being investigated by police.