Rule 12:
Direct free kick
A direct free kick is awarded if a player commits any of the following offences against an opponent in a manner considered by the referee to be careless, reckless or using excessive force:
- charges
- jumps at
- kicks or attempts to kick
- pushes
- strikes or attempts to strike (including head-butt)
- tackles or challenges
- trips or attempts to trip
If an offence involves contact it is penalised by a direct free kick or penalty kick.
- Careless is when a player shows a lack of attention or consideration when making a challenge or acts without precaution. No disciplinary sanction is needed
https://www.thefa.com/football-rule.../football-11-11/law-12---fouls-and-misconduct
It was a careless push. Not reckless or using excess force but careless, with no attempt to win the ball. As others have said it's a soft foul but the onus is with the challenging player to not do it. There are no rules about instigating contact and there was nothing that Lerma did to warrant a push.