The Redcar-born stopper completed his move to Premier League side Bolton Wanderers on Wednesday night, ending seven years at the Riverside.
Boro boss Tony Mowbray now has the job of replacing Wheater with sources in Germany claiming Bayern Munich's 25-year-old defender Maximilian Haas is a target of the Championship side.
McMahon came through the club's academy with Wheater, and the pair were both part of Boro's FA Youth Cup winning side in 2004 under Mark Proctor.
Wheater has two international call-ups to his name when Fabio Capello drafted him into the squad in 2008 to cover for injury, though he is yet to earn his first cap.
But the defender has impressed this season, despite Boro's disappointing position in the Championship, and regular Premier League football could put him back in Capello's thoughts.
New manager Owen Coyle has already backed Wheater to earn a call-up to the senior squad, something McMahon echoes, but the right-back admits it's a blow to lose a key member of the squad.
Obviously it's a massive blow, he's a local lad and what he's done at this club has been brilliant, said the Bishop Auckland-born defender.
Wheats is Wheats, the big daft lad that he is, but everyone loves him here.The fans love him and he's just Wheats and he just comes in and makes you laugh everyday, so he'll be a big miss.
Everyone looks up to him, he's come in as a young lad and done what everyone wants to do. He's gone all the way up, become captain of the first- team and got an England call-up as well, and you can't argue with that.
We know January is January and it could happen any time. It was on and then it was off, then one day it's done and he's gone and we have to get our heads down as a group and we've just got to live life without David Wheater.
We're sad to see him go but that's football I suppose and everyone wishes him all the best.
He'll be a massive success, to think what he's achieved already is brilliant. Obviously like I said, he's a local boy and he's done everything for Boro and now he's moved on I wish him all the best.
I think he can definitely go on to earn a regular call-up in the England squad, which I'm sure he wants to do.
Boro welcome bottom side Preston North End to the Riverside today and with Justin Hoyte set to miss a number of weeks with a hamstring injury, McMahon is likely to fill the gape.
The 24-year-old has been in and out of the Boro side this season, but the former Blackpool and Sheffield Wednesday loanee is hoping a run in the side can help him impress boss Tony Mowbray and cement his side in the starting XI.
Mowbray's side are unbeaten in four in the league, a run which started with the 3-1 win over Preston at Deepdale last month and McMahon hopes the side can continue the form which has lifted them four points clear of the relegation places.
I wouldn't wish injuries on anyone. Justin's my teammate and my friend and I was disappointed for him when he pulled up, but that's what I'm there for and now it's my turn to come in, the right-back said.
I was in the team then out the team, so it's up to me to get my head down, work hard and stay in it.
You just want to play every game, but as a footballer that never happens. When you're out the team you have to keep working hard and that's what I did and now my chance has come again, I've got to take it.
It's a vital game, what's happened has happened now.
We'll train and obviously our heads are on the game against Preston. It's a massive game for us and we need the win.
We're on a good run of late, we've drawn a few but we'd like to have a run but we had a good win on Saturday and we have to build on that and go on a good run.
g Mido has joined Egyptian side Zamalek and claims he will spend the rest of his career with the club.
He played for Ajax this season before leaving Holland and training with Boro but he failed to turn up at Rockliffe Park this week and said of his reutrn to Zamalek last night: I'd like to hang up my boots here.
My only objective is to retire here.
It's time to return back home. I'm here to achieve trophies with the club.''
Source: Northern Echo
Source: Northern Echo