The midfielder made 29 appearances as the Red Devils dropped to seventh in a worst ever showing in the Premier League era, and which resulted in David Moyes's sacking after just 10 months.
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"If anything, I was even more shocked than Carrick. If there is an obvious replacement then fine, but there isn't. He's been under-appreciated in an England shirt and never had the respect he deserves," he wrote in his column for the Daily Mail.
"He's the best holding midfielder England have. He knows the role. He's not constantly wondering whether to stay or go because he plays it, week in, week out.
"But he's not been utilised by the national managers, not given that run of eight, nine, 10 games over a sustained period to settle."
United finished 22 points behind title winners Manchester City and failed to qualify for European competition for the first time in 25 years and Ferdinand feels continued uncertainty was key to the club's failings.
"Unsettled is perhaps a good description of our club season. We weren't playing well and those fluctuations in consistency take their toll. United fans rightly want success and it gets stressful for everyone when it's elusive," he continued.
"The pressure mounted. Questions started to be asked about David Moyes's future as manager. That adds pressure all round. Who might he buy? Who would play? Was he even staying, or going?
"We didn't know. He didn't know. After 27 years of Sir Alex Ferguson, that uncertainty was a shock at United.
"It was always going to be a tough season. Moyes replacing Sir Alex has to be the toughest gig in football. You have to give Moyes some credit for his efforts but it was a tough year — and ultimately Carrick paid. I hope England don't suffer now, too."