I think Lampard is partly easing him in but I think there's more to it than that. He needs to get used to Lampard's demands, Lampard likes his wider players to press well. Pulisic has been the least productive in this area compared to the others in his position, he needs to work harder off the ball. Also, the fact that he struggles to retain the ball at times and sometimes get muscled out of situations which is something he needs to improve on. Compared to Mount, Willian, Hudson-Odoi, and Pedro, Pulisic is the least tactically suited at this moment in time and he hence has to make some improvements to areas of his game to suit the style of play. There's also the fact that Willian, Mount, and Hudson-Odoi are playing well at the moment which means there are naturally fewer chances for him.
I wouldn't say he's struggling, just the fact that he needs to be patient and that he needs to work on a few things. Performance-wise, overall he's been a bit of a mixed bag so far. Unless something big or unexpected happens, it's very unlikely he'll be sold in the summer.