The former West Ham midfielder came on in the second half against his old club in midweek and helped the Red Devils regain control of the match at a time when the Hammers were on top. Boosted by Carrick's introduction, Manchester United successfully protected their first-half lead to win the FA Cup third round replay 1-0.
Now the 31-year-old faces another of his former sides when United travel to White Hart Lane, hoping to avenge their 3-2 defeat to Tottenham earlier this season at Old Trafford.
"I think he's been magnificent," Sir Alex said of Carrick on the club's official website.
"It's his best-ever season at the club and he's commanding that position. He's doing absolutely brilliantly.
"Even when he came on against West Ham, he helped ease us through the last 20 minutes. Good players do that. He came on and settled us down."
Carrick has made 25 appearances for Manchester United this season, scoring once and assisting a further four goals.His improved form has also seen Carrick play his way back into the England squad under Roy Hodgson, after he had effectively retired from international football when refusing to be a standby for Euro 2012.
He made the switch from Spurs to United in 2006, and has since played an integral part in the Red Devils' Premier League and Champions League successes.