De Gea, 20, has come in for criticism after he followed an unconvincing display during United's recent 3-2 Community Shield win over Manchester City with another goalkeeping error against West Brom in Sunday's 2-1 Premier League victory.
However, Phelan, has said: "Young or old, goalkeepers make mistakes. We have got trust in him. He will be a good Premier League goalkeeper."
Smalling, meanwhile, said: "He is making some good saves. His agility and kicking are excellent, we have seen that in pre-season. He is going to be a top goalkeeper."
The Red Devils signed De Gea from Atletico Madrid this summer for around £19 million after experienced shot-stopper Edwin van der Sar retired at the end of last season following six successful years at Old Trafford.