Leeds-born Crooks, who had been with Boro for the final few days of their pre-season tour of the south-west, made his debut in last night’s friendly at Plymouth Argyle, scoring the first goal in a 2-1 success.
The Millers are expected to receive a fee in excess of £1m for the 27-year-old which they can use to kick-start a recruitment drive which has been subdued since they dropped into League One in the summer.
Left-sider Shane Ferguson and former Sheffield United goalkeeper Josh Chapman are their only signings of note so far, though manager Paul Warne said last week he was “really hopeful” of “three or four new faces” arriving soon. St Johnstone right-back Shaun Rooney is one name linked.
Crooks’s signing could pave the way for Lewis Wing, who played impressively alongside him for Rotherham in the second half of last season, to head out on another loan despite manager Neil Warnock talking up his performance in the midweek win over Tavistock.
Warne would love to work with Wing again, but the quality of his performances are likely to keep him in the Championship, with Hull City one of those thought to be interested.
Boro, who have so far signed Joe Lumley, Sammy Ameobi, Uche Ikpeazu and Lee Peltier, are now expected to focus on centre-forwards having released Ashley Fletcher and Britt Assombalonga.
Martin Payero has passed a medical ahead of a move, but the midfielder is representing Argentina at the Olympics and was a second-half substitute against Australia on Thursday.
The versatile Crooks won the Championship’s player of the month award for January after three goals in four games playing in the hole of a 3-5-1-1 formation and it is his goalscoring abilities Warnock will look to exploit.
“I just think he fits exactly what we’re looking for,” he said.
“I think he can contribute 10-15 goals a season and that’s what we need - more goals.”
Crooks scored seven in all last season, and 21 over the two-and-a-half years after joining the Millers from Northampton Town.
His professional debut for Huddersfield Town in August 2014 was his only senior appearance for the Terriers.