Megson started the 19-year-old midfielder ahead of Paris Saint-Germain loanee Grzegorz Krychowiak on Saturday.
Although he told the double Europa League winner that he wasn’t guaranteed a starting place, he said the decision was based on Field’s attributes rather than Krychowiak’s weaknesses.
“It wasn't so much a negative on Grzegorz's part as a positive on Sam's part,” said Megson. “Sam has been training really well.
“I've only been here a short time but in that four months I have seen what he can do in flashes.
“Sam has a range of passing that is probably better than anybody else's at the club at the minute and that is saying a bit after Gareth Barry’s two volleyed passes on Saturday.
“He's big, he can run, he's got a great left foot, he's one the best two-footed players I've seen, he's got a huge amount going for him.
“It wasn't a negative to Grzegorz, it was wanting to get Sam in.”
Field held his own for more than an hour on Saturday, and Albion still haven’t lost a game he has featured in this season and haven’t even conceded a goal while he’s been on the pitch.
However, Megson has challenged him to kick on and establish himself in the first team, because he thinks he’s ready.
“Given the circumstances, I thought he did really well,” said Megson. “He's a teenager and if he can just grow into himself in terms of confidence and have that little belief in himself.
“He's got to take what he does on the training pitch and put it on the pitch and at times on Saturday he did that, but not enough.
“He has now got to deliver on the promise that everyone knows he has got to being a recognised starting player in the Premier League. He has to grow into it.
“For a midfield player he's got to be getting up there and scoring - because he can finish - and then getting back for the other side of it.
“It wouldn't surprise me if he went on to have a real top-class career and hopefully he can get it going sooner rather than later.”
Krychowiak arrived in the summer on a season-long loan from PSG and the Baggies are stumping up the cash for his full £108,000-a-week wage packet.
The Polish midfielder started brightly in the West Midlands but is yet to win a game with the Baggies and has looked disinterested in recent weeks.
Megson said reputation is not enough to earn a starting place, but said he’d seen the right reaction from the Pole.
“I don't think the fact you have played for certain football clubs or what you have done in the past has any bearing on what is happening in the here and now,” he said.
“You just go on what you see. Grzegorz I am sure will be disappointed that he didn't start as indeed anyone else would but sometimes people have to come out of the firing line and someone else goes in. But it is not always - and wasn't in this case - a negative.
“He was alright and when I say alright, the right reaction was he is disappointed. He hasn't come in here to sit on the bench but when he came there were no guarantees from Tony (Pulis) he would play because he has played for some really big clubs.
“He has had that opportunity, he didn't have it on Saturday and we will see what happens on Tuesday.”
On Saturday, Albion ran further than any other team have done in the Premier League this season (121.21km collectively) and Megson admitted he might need to make changes for the Newcastle game.