Elmohamady worked under Agnew for two-and-a-half years at the KCOM Stadium before the latter swapped his role as assistant to former Hull manager Steve Bruce for a similar post at the Riverside in December 2014.
Agnew, now in temporary charge at Boro, will return to the KCOM for a classic 'six-pointer' in search of his first win since Aitor Karanka was sacked last month, and Elmohamady admitted all goodwill has been placed on hold.
"I think Aggers is one of the best coaches I have worked with through all my career," the Egypt international said
"He was fantastic here and has the chance to go back to Middlesbrough and deserves the chance.
"He's a fantastic man and a fantastic coach, b ut hopefully (on Wednesday) we can beat him because we need the three points."
Second-bottom Boro, without a league win since December 17, arguably need them more, but Hull can climb out of the relegation zone for the first time since October with victory if Swansea fail to beat Tottenham at the Liberty Stadium.
The Tigers are chasing a fifth home win in six under head coach Marco Silva and if the Portuguese can extend his unbeaten home league run as manager to 40 matches, his side's momentum could prove decisive in their fight for survival.
"Of course, everyone wants to be out of the bottom three, especially (on Wednesday) it's a big game for us," Elmohamady added.
"With eight games coming, it's massive
They're all cup finals, but I think the most important game is this one.
"To get out of the bottom three, it will give everyone more confidence going forward
It's going to be a massive game."
Silva, whose unbeaten home record spans spells at Sporting Lisbon and Olympiakos before his arrival at Hull, said another three points on the back of Saturday's win against West Ham outweighed the significance of climbing clear of the bottom three.
"It's important, but the most important thing for us is to be out of the zone at the end of the season," he said.
"We'll try to do our best to win the game and if you get out of the zone it's a good sign, but best for us is if we're out at the end of the season.
"I prepare my players to win and take points not only at home, but of course they are great numbers.
"It gives us all confidence and it's clear our team is more comfortable at home
All teams are of course and it's important we keep these numbers, but it's important as well we improve away."
Silva will again be without Tom Huddlestone, who serves the second game of a three-match ban, but fellow midfielder Evandro will return to contention after missing the last five games due to a calf injury.
Defender Omar Elabdellaoui missed Saturday's defeat of West Ham because of a back problem and will play no part on Wednesday night or at Manchester City on Saturday.
Silva has no new injury worries, but Dieumerci Mbokani, Ryan Mason, Moses Odubajo and Will Keane all remain sidelined.
Source : PA