Not only does Lackey suck, he'd be pitching on 3 days rest. Chen would be on 5 days rest. Chen actually had a decent year with the Royals, going 12-8 with a 3.98 ERA.
Chen didn't face the Rays at all this year, but in 2010, he started against them once and it was a complete game, 2 hit shutout. But in '09 against them, he had a 6.2 inning, 2 earned run game in which he allowed 5 hits and 3 walks.
The current Rays aren't too familiar with him, as they only have 64 total at-bats facing him. 28 of those at-bats belong to Johnny Damon, who hits .464 off him. BJ Upton is 1/12 in his career against Chen. Other than that, though, no Ray has more than 5 at-bats against him. No Ray has more than 1 hit, either. Pitchers tend to have the advantage when hitters aren't familiar with them.
The Rays are more familiar with Lackey (235 total at-bats). They hit .294 off him. Damon is 22 for 60 (.367), Ben Zobrist is 8 for 20 (.400). Lackey was 1-2 vs. Tampa Bay this season, with a 6.46 ERA. Last year he was 2-2, with a 6.26 ERA. This isn't a good matchup for him.
Chen was hit or miss in September. On the 18th, he made a 5.1 inning, 4 earned run start against the White Sox. Then on the 23rd, he went 8 innings against the same White Sox, allowing only 1 run off 2 hits. He was 2-3 in the month, with a pair of brilliant 8 inning starts, and 3 mediocre 4+ run starts.
Lackey has hardly been better down the stretch. 0-2 record with a 9.13 ERA in the month. He had a 3 inning, 5 earned run start against the Rays on the 9th. Like Chen, he was inconsistent. Unlike Chen, he made 3 atrocious starts in September, not 3 mediocre starts. And his 2 "good" starts were just good, not excellent, which Chen's were.
It's not that Bruce Chen is a good pitcher. It's just that John Lackey sucks. I know the entire pitching staff will be available in the bullpen in case Lackey falters, but I think Chen is less likely to outright blow the game. And with Chen, there's at least a possibility of a very good start.
Neither option is very pleasant, but I feel like Chen is the lesser of two evils.