Manchester United midfielder Michael Carrick has undergone an operation for an irregular heart rhythm, he said on his Twitter feed on Thursday.
"After feeling strange in the second half of the Burton game in September, I underwent a series of tests," he said.
"It turned out to be due to an irregular heart rhythm which has been fully investigated and I had a procedure called a Cardiac Ablation."
The 36-year-old said he was making the statement in response to questions about why he had not been playing.
Carrick said he has had to rebuild his fitness and aimed "to be back in contention soon".
"I'm feeling fine now. I'd like to reassure everyone that I'm healthy and back training hard with the team."
Carrick played in four pre-seasons games for United but did not feature again until the EFL tie against Burton.
Carrick made his debut for West Ham United in 1999, joining Tottenham Hotspur and then United in 2006. He has played more than 300 times for the club and been capped 34 times by England.
In a cardiac ablation, tissue in the heart that triggers or sustains an abnormal rhythm is scarred or destroyed to return the organ to a normal rhythm.
Guardiola was logical choice to be United boss, says Cantona
Manchester City's Pep Guardiola should have been the "logical" choice to take charge of Manchester United as current manager Jose Mourinho's defensive approach belies their attacking identity, former forward Eric Cantona has said.
United great Cantona helped transform former manager Alex Ferguson's side into a force in English soccer, being the focal point of their attack for five years in the 1990s, winning four Premier League titles and two FA Cups.
Although Mourinho guided United to two titles after taking over from Louis van Gaal last year, he has been criticised for his caution in games while Guardiola's free-flowing City have won new admirers for their ruthless run to the top of the table, including Cantona.
"I cannot understand why United would take a manager who plays in a defensive way. I love Mourinho, but I prefer to watch Barcelona play," Cantona told BBC Radio 5 Live.
"I prefer the way Guardiola plays, and I would prefer him as manager of Manchester United, it's more logical. I don't watch ManCity because it's ManCity but I would love to watch the team managed by Guardiola."
City have 17 wins in a row this season and have pumped in 54 goals in an unbeaten run of 19 games in all competitions while United have lost four times and have scored 11 goals fewer to sit in second place in the league behind their neighbours.
"He (Mourinho) has a lot of charisma, has won a lot of things and will win a lot of things. But, the football I played, and Alex Ferguson played, Matt Busby before, it was creative.
"I loved (Johan) Cruyff when he was manager at Barcelona, I loved Ajax in the 1970s, I grew up with this kind of football and the football we played with Ferguson was very creative and more Manchester United.
"It's why I prefer the way Guardiola plays and I would prefer to have him as manager of Manchester United."
Moyes makes his first point as Hammers draw with Leicester
West Ham registered their first Premier League point under new manager David Moyes after battling back to draw 1-1 with Leicester City at the London Stadium on Friday.
With Mark Noble dropped from midfield, the Hammers got off to the worst possible start after seven minutes of Moyes's home debut when Marc Albrighton stabbed the ball home after the defence failed to deal with a Jamie Vardy cross from the left.
Moyes will be delighted his team did not buckle on a chilly night and, spurred on by a supportive crowd, they equalised just before the interval when Cheikhou Kouyate headed in off Danny Simpson from a Manuel Lanzini corner, West Ham's first opening-half goal in nine games.
Andre Ayew replaced the impressive Marko Arnautovic halfway through the second half and thought he should have been awarded a penalty for Harry Maguire's challenge. Yet the claims were waved away, leaving West Ham, who showed more effort than quality, without a win in six games and still in the drop zone in 18th place.