Garnacho was sold to Chelsea in August as United boss Ruben Amorim backed Amad as their prime winger.
Reflecting on the pair's form so far this season, Sharpe has no doubts Amad is proving himself the superior talent.
He told Tribalfootball.com: "Oh absolutely I would 100% agree with that. I think Garnacho, as much ability as he's got, I think his decision-making is a little bit poor. I was one of those people that thought it might be worth a gamble letting him go.
"I think the decisions he gave the manager is now starting to just take shape and show that it was the right decision."
On seeing Amad fielded on the right-side, Sharpe continued: "Yeah, I think it's really good. I think he works well.
"I think he works really well with (Bryan) Mbeumo. I think it is quite handy that he does have the left foot. I think he's quite happy to go down the outside at times, but it's nice that he can cut in and cause some trouble when we're in that offensive position."
Asked where he feels Amad can improve, Sharpe continued: "I think defensively, positionally, and his decision making defensively. I think he's done a really good job so far. It's not his natural position, there is a lot more responsibility defensively for him, but I think he's doing a good job.
"I think his work ethic, his willingness to learn and become a better player, I think is obvious. You don't really want him so far back to defend so much. But I think he does a really good job. And so far, I don't think he's not let anybody down. I think he's still been one of our best players."
Another winger needed?
However, Sharpe admits there would be concern if Amad is injured. The Premier League title winner says United would do well adding another senior wing-back to increase the options for Amorim.
He explained, "The manager doesn't really play out and out wingers, so I don't think you really need a depth of wingers when you're playing wing-backs and you're playing inside forwards.
"I think there's probably a lack of depth in quality wing-back. I think (Diogo) Dalot is a little bit inconsistent. I think he gives the ball away a little bit too cheaply and defensively can be a little bit irresponsible at times.
"(Patrick) Dorgu has struggled with a bit of form and with a couple of injuries this season. So you could possibly say that there's a lack of depth in those positions. And that's probably why Amad is playing so regularly because as a winger and a forward-thinking player, you do neglect defensive responsibilities at times.
"But I think he's done a great job. Maybe they need another winger on the other side to play the wing-back role."
- Lee Sharpe was speaking to Tribalfootball on behalf of Adventure Gamers