Lucas Giolito is easily the most mentioned starting pitcher trade target this year that fits the criteria here as he made the AL All-Star team back in 2019. He hasn't been amazing this season, but Giolito is still striking out batters at a 9.7 K/9 rate and could benefit from a change of scenery.
There is no question that Giolito will be available at the deadline as the White Sox are one of the few guaranteed sellers this year and he is a pending free agent. However, the biggest hurdle to Boston acquiring him is that this is definitely a seller's market and Chicago is going to be fielding a LOT of strong offers for Giolito from all over the league.
No one is confused as to why Justin Verlander is eligible for consideration here. The Mets collapse in the first half this year has been well documented and they now find themselves closer to having the worst record in the NL this year than they are to a playoff spot. Also, Verlander's track record as one of the best pitchers of his generation is well-documented and he has pitched much better lately.
It does sound like the Mets are at least entertaining the idea of moving Verlander, but there are some problems here. First, he has a full no trade clause and there is no guarantee whatsoever that he is going to be willing to go to Boston or anywhere else for that matter. The other issue is that the Mets are going to want to try and compete next season and Verlander is under contract through 2024 with a vesting option for 2025. Not only is signing up for that level of a financial commitment to an older pitcher not a no-brainer for Boston, but the Mets may also not be willing to actually move him unless they get big league ready talent back for him so they can make a run in 2024.
Marcus Stroman is an interesting case because while he is a two-time All-Star, the Cubs are fringe playoff contenders this year despite being three games below .500. However, at five games out of the wild card and 6.5 games out in the NL Central, the smart money does seem to be that the Cubs will sell at the deadline and Stroman could command a premium for them as one of the better starting pitchers on the market.
Stroman is great at getting a lot of groundballs and usually throws a lot of strikes, but he has seen his walk rate spike a bit this season. His contract also muddies the water a bit as he has a player option for 2024 and while he seems to love living in Chicago, it is pretty unclear as to whether or not he will exercise that option. That makes placing a trade value on him pretty tricky given that teams won't know how long he will actually be with the team.