Jose Mourinho continues to play down the London club's title hopes, but they have looked every inch contenders during a 10-game unbeaten run in the top flight.
Eden Hazard scored a hat-trick as Newcastle United became the latest side to suffer as a result of Chelsea's omimous form, with Alan Pardew's side beaten 3-0 at Stamford Bridge on Saturday.
Arsenal's 5-1 defeat at Liverpool and Manchester City's draw against Norwich City ensured Chelsea took over at the summit with 13 games remaining.
Not since a 3-2 reverse at Stoke City early in December have Chelsea suffered a Premier League defeat and they have not conceded away from home in four league games since then.
Mourinho's men will be expected to continue their impressive form at The Hawthorns against a West Brom side that have dropped into the relegation zone.
Chelsea needed a controversial last-gasp Hazard penalty to salvage a 2-2 draw against Albion at Stamford Bridge in November, after goals from Long and Stephane Sessegnon looked to have given the visitors a shock win during Steve Clarke's tenure.
Albion beat Chelsea at home last season courtesy of goals from Long and Peter Odemwingie, but the two strikers have both recently left the club.
Pepe Mel's side slumped to a 3-1 defeat at Crystal Palace on Saturday to slip into the bottom three and are now without a win in six games across all competitions.
The West Midlands club have drawn their last two home games against Everton and Liverpool, but victories have proved elusive and Spaniard Mel is braced for a battle to avoid the drop.
He said: "The first thing is to try and win the game on Tuesday, and if we want to stay in this league we must play like we did in the second half against Liverpool last weekend and the second half on Saturday (against Palace)."
Chelsea captain John Terry was not risked against Newcastle after picking up a knock, but the defender is expected to feature on Tuesday.
Mourinho has also revealed that striker Fernando Torres could make his comeback at Albion after a lay-off with a knee problem.
Albion could again be without Jonas Olsson (hamstring) and former Chelsea striker Nicolas Anelka (knee), while Sessegnon (groin) is definitely out.
Striker Thievy Bifouma could earn a first West Brom start, having scored less than a minute after coming on as a substitute to make his debut at Palace.
