Leeds-born Crooks has been with Boro for the final few days of their pre-season tour of the south-west and should now make his debut in the final friendly of it, at Plymouth Argyle on Friday eveing.
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 keeper 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 arrival 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 pre-season friendly 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 in the summer.
Martin Payero has passed a medical ahead of a move, but the midfielder is currently in Tokyo representing Argentina at the Olympics. Payero was a second-half substitute against Australia on Thursday.
The versatile Crooks won the Championship's player of the month award for January after scoring 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’ve watched Matt for many, many years from playing centre-half at Huddersfield," he said. "He’s played all over the show really.
“I just think he fits in with exactly what we’re looking for.
“I think he can come in and contribute 10-15 goals a season and that’s what we need - more goals.”
Crooks's January goals raised hopes that the Millers might avoid an instant return to League One but a controversial red card - ironically in defeat to Middlesbrough - and the subsequent three-match ban hampered their chances, and they went down on the final day.
Crooks scored seven goals in all last season, and 21 over the course of two-and-a-half years since joining from Northampton Town. He made his professional debut for Huddersfield Town in August 2014, but it was his only appearance for the Terriers.
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