Sutton defender Ben Goodliffe has been ruled out for six months with a knee injury.
Goodliffe, 23, limped off during the 1-1 League Two draw at home to Newport County on 30 July and scans have revealed a ruptured posterior cruciate ligament.
Sutton boss Matt Gray said Goodliffe's "attitude and character" and the club's medical staff would aid his recovery.
"Ben's been a major player for us and it's a huge blow to lose him," he said.
"I know all parties will work extremely hard to see him back in a Sutton shirt as soon as possible.''
Goodliffe was Sutton's player of the year for the 2021-22 season, helping the club to an eighth-placed finish and a place in the EFL Trophy final.
orward Omar Bugiel and midfielder Will Randall are among 10 players who have been released by Sutton United after the end of the League Two campaign.
The pair were the joint-top scorers for the U's this term with six goals each.
Defender Cody Rowe and forward Donovan Wilson, who - like Bugiel and Randall - both featured more than 30 times this season, will also leave the club.
Filip Chalupniczak, Roman Charles-Cook, Luke Gambin, Adam Lovatt, Matt Ridley and Kwame Thomas round off the exits.
Sutton United have signed goalkeeper Steve Arnold and defender Omar Sowunmi.
Arnold, 33, moves to the League Two side as a free agent having left Southend United at the end of his contract following a two-year spell.
He has spent time at Northampton Town, Gillingham and Stevenage and is a former England C player from his time in non-league football.
Sowunmi, 27, also joins as a free agent, having spent the past two years in the National League with Bromley.
The former Ipswich Town academy player has also spent time at Colchester United and Yeovil Town.
"'Steve's someone we know well, as I worked with him early in my career as an assistant manager at Eastleigh," Sutton manager Matt Gray told the club website.
"He is in his prime as a goalkeeper and will provide excellent competition for Jack Rose. who had such a good season last season.
"'Omar's had a great couple of seasons with Bromley and nearly won promotion with them last season.
"He's got huge defensive qualities and is another player in the prime of his career, and will be good competition for Ben Goodliffe and Louis John.''
N'Guessan, who is 24, helped Ebbsfleet win National League South last season and began his career at Blackpool.
He too joins on a free transfer after his contract came to an end.
Christian's a player we've had our eyes on for a while. He's done brilliantly with Ebbsfleet and has the potential to make a real impact in League Two.''
Sutton manager Matt Gray told the club website.
Defender Ryan Jackson will join Sutton United when his contract at Cheltenham Town expires at the end of the month.
The 32-year-old right-back made 43 appearances for the Robins after joining from Gillingham last summer.
Jackson has previously had spells with AFC Wimbledon, Macclesfield Town, Newport County and Colchester United.
"He's got vast experience, is a tremendous character and will bring strong competition to our back line," boss Matt Gray told the club website.
The League Two club have not disclosed the length of Jackson's contract.
Sutton United have signed Harry Smith after the striker was released by League Two champions Leyton Orient.
The 28-year-old made 10 appearances for the O's last season and also spent time on loan at Exeter City and Barnet.
In total he scored 15 goals in 55 appearances for the east London club during a two-year stint at Brisbane Road.
Smith has previously had spells with Folkestone Invicta, Millwall, Macclesfield Town and Northampton Town.
Sutton, who finished 14th in the fourth tier last season, have not disclosed the length of his contract.
Sutton United have signed winger Omari Patrick following his exit from Carlisle United.
The 27-year-old scored 21 goals in 109 appearances for Carlisle during two spells, but turned down the offer of a new deal this summer.
He scored a crucial goal during last season's League Two play-off final as the Cumbrians beat Stockport on penalties and sealed promotion .
Patrick's former clubs include Bradford City, Yeovil Town and Burton Albion.
Sutton United have signed Shrewsbury Town forward Aiden O'Brien on loan.
The 29-year-old has agreed a deal to move to the League Two side until January, returning to the capital three years after leaving Millwall.
He has scored 66 goals in 355 senior appearances and has also spent time at Sunderland and Portsmouth.
He joined the Shrews a year ago but made just six substitute appearances in League One before moving to Gillingham on loan in January.
The former Republic of Ireland striker scored two goals in 14 League One games for the Gills last season.
''I worked with Aiden when he was at Aldershot on a loan spell from Millwall, and know what a good character he will be in the dressing room and on the pitch," Sutton boss Matt Gray told the club website.
"He's gone on to have a great career with Millwall, Sunderland and Portsmouth as well as playing internationally, and I'm delighted to get the chance to work with him again.
"He's mainly a striker, but can also play wide and will give us quality, and options, in our front line.''
League Two club Sutton United have signed Wycombe Wanderers striker D'Mani Mellor on loan until January.
The 22-year-old has scored one goal in 13 outings for the Chairboys since joining from Manchester United in July last year.
He spent the second half of last season on loan with Rochdale, making 19 appearances as Dale were relegated to the National League.
Mellor is eligible to make his Sutton debut at Barrow on Saturday.
Sutton United have signed goalkeeper Dean Bouzanis from Reading and winger Dion Pereira from Luton on loan deals.
Bouzanis, 32, has moved to South London until January while 24-year-old Pereria has joined for the rest of the season.
Bouzanis previously played for Sutton for two seasons from 2020 and helped them win the National League and reach the EFL Trophy final the next season.
Pereira has made one Championship appearance and played twice in the EFL Cup for Luton.
He has also had loan spells at Yeovil and Bradford.